cross-dehydrogenative coupling for the intermolecular c o ... · the c–h bond of arene, the...

92

Cross-dehydrogenative coupling for the intermolecularC–O bond formationIgor B. Krylov, Vera A. Vil’ and Alexander O. Terent’ev*§

Review Open Access

Address:N. D. Zelinsky Institute of Organic Chemistry, Russian Academy ofSciences, Leninsky Prospect 47, Moscow, 119991, Russia

Email:Alexander O. Terent’ev* - [email protected]

* Corresponding author§ Tel +7-916-385-4080

Keywords:acyloxylation; alkoxylation; C–H functionalization; C–O bondformation; cross-dehydrogenative coupling; oxidative cross-coupling

Beilstein J. Org. Chem. 2015, 11, 92–146.doi:10.3762/bjoc.11.13

Received: 14 October 2014Accepted: 31 December 2014Published: 20 January 2015

Associate Editor: S. You

© 2015 Krylov et al; licensee Beilstein-Institut.License and terms: see end of document.

AbstractThe present review summarizes primary publications on the cross-dehydrogenative C–O coupling, with special emphasis on the

studies published after 2000. The starting compound, which donates a carbon atom for the formation of a new C–O bond, is called

the CH-reagent or the C-reagent, and the compound, an oxygen atom of which is involved in the new bond, is called the

OH-reagent or the O-reagent. Alcohols and carboxylic acids are most commonly used as O-reagents; hydroxylamine derivatives,

hydroperoxides, and sulfonic acids are employed less often. The cross-dehydrogenative C–O coupling reactions are carried out

using different C-reagents, such as compounds containing directing functional groups (amide, heteroaromatic, oxime, and so on)

and compounds with activated C–H bonds (aldehydes, alcohols, ketones, ethers, amines, amides, compounds containing the benzyl,

allyl, or propargyl moiety). An analysis of the published data showed that the principles at the basis of a particular cross-dehydro-

genative C–O coupling reaction are dictated mainly by the nature of the C-reagent. Hence, in the present review the data are classi-

fied according to the structures of C-reagents, and, in the second place, according to the type of oxidative systems. Besides the

typical cross-dehydrogenative coupling reactions of CH- and OH-reagents, closely related C–H activation processes involving

intermolecular C–O bond formation are discussed: acyloxylation reactions with ArI(O2CR)2 reagents and generation of O-reagents

in situ from C-reagents (methylarenes, aldehydes, etc.).

92

IntroductionThe development of methods for the cross-dehydrogenative

coupling (CDC; or oxidative cross coupling) is an important

field of modern organic chemistry. These terms commonly refer

to reactions, in which two different molecules are linked by a

new bond accompanied by the elimination of a hydrogen atom

from each molecule (Scheme 1) [1-15]; however, these terms

are also applied to a large number of various reactions with

oxidants, which involve the intermolecular formation of new

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Beilstein J. Org. Chem. 2015, 11, 92–146.

93

bonds between the starting molecules. For instance, such reac-

tions involve the oxidation of several C–H bonds, the elimina-

tion not only of hydrogen atoms but also of other moieties from

the starting molecules, the addition at C–C multiple bonds, and

so on.

Scheme 1: Cross-dehydrogenative coupling.

The cross-dehydrogenative coupling can be employed to form a

new bond with high atomic efficiency and does not require ad-

ditional synthetic steps for the introduction of functional groups

(for example, such as -Hal, -OTf, -BR2, -SnR3, -SiR3, -ZnHal,

-MgHal) into molecules necessary in other cross-coupling reac-

tions. Therefore, the cross-dehydrogenative coupling is a

promising approach to the minimization of byproduct forma-

tion and the reduction of the number of steps of the organic syn-

thesis [1-15].

Studies of the cross-dehydrogenative coupling are not only of

practical but also of fundamental interest because new aspects

of the reactivity of organic compounds have to be found for the

performance of these reactions. The prediction of the condi-

tions necessary for the efficient cross-dehydrogenative coupling

is an important problem that requires an understanding of the

mechanisms of these processes.

Among cross-dehydrogenative coupling reactions, C–C

coupling reactions have been studied in most detail [1-14],

whereas the C–O coupling is less well-known (Scheme 2). We

present the first systematic review of the main approaches to the

cross-dehydrogenative C–O coupling. The starting compound,

which donates a carbon atom for the formation of a new C–O

bond, is called the CH-reagent or the C-reagent, and the com-

pound, an oxygen atom of which is involved in the new bond, is

called the OH-reagent or the O-reagent.

Scheme 2: Cross-dehydrogenative C–O coupling.

Alcohols and carboxylic acids are most commonly used as

O-reagents; hydroxylamine derivatives, hydroperoxides, and

sulfonic acids are employed less frequently. In the case of

O-reagent PhI(O2CR)2, dehydrogenated carboxylic acid

RCO2H is preliminary included in the oxidant, so C–H acyloxy-

lation with PhI(O2CR)2 can be considered as the second stage

in a two-step cross-dehydrogenative C–O coupling. The forma-

tion of a new C–O bond generally takes place with the involve-

ment of an O-nucleophile, an O-radical, or an O-electrophile. In

the oxidative coupling with O-reagents as nucleophiles, electro-

philic intermediates that are generated from C-reagents are

prone to side transformations. Therefore, O-reagents are often

used in excess amounts. The C–O coupling reactions involving

O-centered radicals are generally performed under severe

conditions. In addition, O-radicals are highly reactive and

unstable, and the reactions with these radicals are often non-

selective and are accompanied by the formation of alcohols,

carbonyl compounds, and fragmentation products. The exam-

ples of the C–O bond formation between two molecules using

O-electrophiles are rare; electron-deficient peroxides with a

specific structure can act as O-electrophiles [16-18]. These

processes are not consistent with general Scheme 2 of the cross-

dehydrogenative C–O coupling and are not considered in the

present review.

The cross-dehydrogenative C–O coupling reactions are carried

out using different C-reagents, such as compounds containing

directing functional groups (amide, heteroaromatic, oxime, and

so on) and compounds with activated C–H bonds (aldehydes,

alcohols, ketones, ethers, amines, amides, compounds

containing a benzyl, allyl, or propargyl moiety). An analysis of

the published data showed that the principles at the basis of a

particular cross-dehydrogenative C–O coupling reaction are

dictated mainly by the nature of the C-reagent. Hence, in the

present review the data are classified according to the struc-

tures of C-reagents, and, in the second place, according to the

type of oxidative systems. Since structurally different

OH-reagents are often involved in C–O coupling reactions of

the same type, the classification according to the structures of

O-reagents is inconvenient and was not applied.

Some cross-dehydrogenative C–O coupling reactions are curso-

rily described in reviews on the oxidative C–heteroatom bond

formation without the use of metal compounds [15], the Pd(II)-

catalyzed oxidative C–C, C–O, and C–N bond formation [3],

the transition metal-catalyzed etherification of unactivated C–H

bonds [19], the Pd(II)-catalyzed oxidative functionalization at

the allylic position of alkenes [20,21], the oxidative functional-

ization catalyzed by copper compounds to form C–C, C–N,

C–O, C–Hal, C–P, and N–N bonds [10], the Bu4NI/t-BuOOH

oxidative system [22], selective functionalization of molecules

[23], the oxidative esterification and oxidative amidation of

aldehydes [24], and the transition metal-catalyzed radical oxida-

tive cross-couplings [13].


94

Scheme 3: Regioselective ortho-acetoxylation of meta-substituted arylpyridines and N-arylamides.

The present review summarizes primary publications on the

cross-dehydrogenative C–O coupling, with special emphasis on

the studies published after 2000. The focus is on the reactions

described by general Scheme 2.

Review1 C-Reagents containing directing groups incross-dehydrogenative C–O couplingNitrogen-containing moieties (amide, pyridine, oxime, etc.) are

most commonly used as directing groups, which are respon-

sible for the regioselectivity of the C–O coupling. Most trans-

formations of this type are catalyzed by Pd(II) compounds.

Examples of the use of copper and ruthenium compounds as

catalysts were also reported. It is commonly assumed that the

reaction proceeds via the C–metal bond formation accompa-

nied by the C–H bond cleavage assisted by the directing group

of the substrate, which forms a complex with the metal ion. The

mechanism of this type of reactions was considered in detail in

the publications [25-32].

1.1 Reactions involving C(sp2)–H bonds of aromaticC-reagentsIn one of the first publications on the preparative introduction of

the –OR group into CH-reagents containing directing groups,

8-methylquinoline, 2-arylpyridines, N-phenylpyrazole, azoben-

zene, and benzylideneaniline were subjected to the acetoxyla-

tion using the Pd(OAc)2/PhI(OAc)2 system [33]. More recently,

reactions involving the same and some other directing groups

were studied in more detail. In most of the studies, Pd(OAc)2

was used as the catalyst, and PhI(OAc)2 or peroxides served as

the oxidants.

The regioselectivity of the ortho-acetoxylation of meta-substi-

tuted arylpyridines and N-arylamides 1 was studied [34]. The

acetoxylation occurs mainly at the sterically more accessible

para-position relative to the substituent R to form product 2.

The lowest regioselectivity (2:3 = 6:1) was observed in the case

of R = F (Scheme 3).

The pyridine moiety served as the directing group in many

works to accomplish the ortho-acyloxylation of arenes 4 giving

products 5 (Table 1). The reactions were catalyzed by copper,

palladium, or rhodium salts. Carboxylic acids or their salts, as

well as aldehydes, methylarenes, arylethylenes, and aryl-

acetylenes were used as precursors of the acyloxy fragment.

The cross-dehydrogenative C–O coupling with 2-arylpyridines

4 proceeds also in the presence of the Cu(OAc)2/O2 system [40]

and under electrochemical oxidation in the presence of Pd(II)

salts [41].

The pyrimidine (acetoxylation of 6 to form 7) [42], benzoxa-

zole (acetoxylation of 8 to form 9) [43], benzimidazole (alkoxy-

lation of 10 to form 11) [44], and triazole (acyloxylation of 12

to form 13 [45], alkoxylation of 14 to form 15 [46]) moieties

were also used as directing groups for the ortho-acyloxylation

and alkoxylation of arenes (Scheme 4).

The pyridine, pyrimidine, or pyrazole moiety serves as the

directing group in the oxidative ortho-alkoxylation of arenes 16

with the Cu(OAc)2/AgOTf/O2 system giving coupling products

17 (Scheme 5) [47]. It is supposed that copper is inserted into

the C–H bond of arene, the resulting Cu(II) complex is oxidized

by silver(I) ions to Cu(III) complex 18, and the C–O bond is

formed via reductive elimination. The drawbacks of this method

are the use of large amounts of silver triflate and alcohol and the

high temperature of the reaction.

The Pd(OAc)2/persulfate system was used in the ortho-alkoxy-

lation of arylnitriles 19–20 [48], N-methoxybenzamides 21 [49],

and acetanilides 22 [50] and in the ortho-acetoxylation of

acetanilides 22 [51] and sulfoximines 23 [52] to prepare cross-


95

Table 1: ortho-Acyloxylation of arenes 4 directed by pyridine moiety.

Conditions RX Yield of 5, % Ref.

[Rh(cod)Cl]2 (5 mol %)P(Cyclohexyl)3·HBF4 (7.5 mol %)CuI (40 mol %)phenanthroline (10 mol %)N-methylpyrrolidone, 130 °C, 36 h

RCOOH (0.5 or 2 equiv); R = Ar, CH=CHPh,Me

43–85 [35]

Pd(OAc)2 (10 mol %)CuI (1 equiv)Ag2CO3 (1 equiv)O2dichloroethane, 80 °C

RCOOH; R = Ar, Me 53–78 [36]

Cu(OTf)2K2S2O8toluene, 130 °C, 24 h

RCOONa; R = Ar 35–86 [37]

Cu(OAc)2 (10 mol %)t-BuOOH (2–4 equiv)PhCl or without a solvent, 135 °C, 24–40 h

RCHO or ArCH3; R = Ar, n-Bu, n-Pr 20–56 [38]

Cu(OAc)2 (20 mol %)t-BuOOH (10 moles per mole of arene)PhCl, 120 °C, 10–22 h

ArCH=CH2 or ArC≡CH (2 equiv) 48–81 [39]

dehydrogenative C–O coupling products 24–30 (Scheme 6).

The alkoxylation of 1-naphthonitrile 20 occurs not at the ortho-

position but at the 8-position of the aromatic system to give

product 26. The acetoxylation takes place under more severe

conditions compared with the alkoxylation. The acetoxylation

employing the S-methyl-S-2-pyridylsulfoximine moiety as the

bidentate directing group can be performed at lower tempera-

ture (50 °C instead of 100 °C, as in the case of CH-reagents 22

and 23) [53].

The ortho-acetoxylation and methoxylation of O-methyl aryl

oximes 31 with Pd(OAc)2 combined with such oxidants as

oxone, potassium persulfate, and (diacetoxyiodo)benzene

(Scheme 7, coupling products 32 and 33) occur under similar

conditions [54]. N-Phenylpyrrolidin-2-one (34) and (3-benzyl-

4,5-dihydroisoxazol-5-yl)methyl acetate (35) react in a similar

fashion to afford products 36–38. Related ortho-acetoxylation

reactions of the aryl group of methoxyimino-2-aryl acetates

[55], 2-methoxyimino-2-arylacetamides [55], and O-acetyl aryl

oximes [56] in the presence of the Pd(OAc)2/PhI(OAc)2 system

were described.

The ruthenium-catalyzed ortho-acyloxylation of acetanilides 39

with carboxylic acids in the presence of AgSbF6 and ammoni-

um persulfate afforded products 40 (Scheme 8) [57]. This

method can be used for the selective replacement of one of the

two hydrogen atoms in the ortho-position of acetanilide; the

molar ratio of the C- and O-reagents is close to stoichiometric.

The acetoxylation (product 42) and methoxylation (product 43)

of N-(2-benzoylphenyl)benzamides 41 at the ortho-position of

the benzamide moiety of the substrate were performed using

Pd(OAc)2 combined with PhI(OAc)2 as the oxidant (Scheme 9)

[58]. The alkoxylation of N-tosylbenzamides 44 in the presence

of the same oxidative system takes place at room temperature

and gives products 45 [59]. The reactions of benzamides

containing the nitro group at the ortho-position are most diffi-

cult to perform [58,59].

The Pd(OAc)2/PhI(OAc)2 system was also employed to accom-

plish the ortho-alkoxylat ion of azoarenes 46 [60],

2-aryloxypyridines 47 [61], picolinamides 48 [62], and N-(1-

methyl-1-(pyridin-2-yl)ethyl)amides 49 [63], resulting in the

formation of products 50–54 (Scheme 10).

The ortho-acetoxylation of compounds containing picolin-

amide and quinoline-8-amine moieties (55 and 56, respectively)

with the Pd(OAc)2/PhI(OAc)2 system in a AcOH/Ac2O mix-

ture, resulting in the formation of products 57, 58, was

performed at higher temperature (150 °C, Scheme 11)


96

Scheme 4: ortho-Acyloxylation and alkoxylation of arenes directed by pyrimidine, benzoxazole, benzimidazole and triazole groups.

Scheme 5: Cu(OAc)2/AgOTf/O2 oxidative system in the ortho-alkoxylation of arenes.


97

Scheme 6: Pd(OAc)2/persulfate oxidative system in the ortho-alkoxylation and acetoxylation of arenes with nitrile, amide, and sulfoximine directinggroups.

compared with the alkoxylation of structures 46–49 [64]. Under

similar conditions, aryl phosphates and benzyl phosphonic

monoacids were subjected to ortho-acetoxylation in the pres-

ence of the Pd(OAc)2/PhI(OAc)2 system; (diacetoxy-

iodo)benzene served as the source of the acetoxy group [65].

The quinoline-8-amine directing moiety was used for copper-

catalyzed aerobic ortho-aryloxylation and alkoxylation of

arenes 59 with electron donating or electron withdrawing

substituents to afford products 60 [66] (Scheme 12). The

method is applicable to a wide range of OH-reagents; the molar

ratio OH-reagent/CH-reagent was 1:1 for aromatic OH-reagents

and 5:1 for aliphatic alcohols. Different organic and inorganic

bases were used depending on the structures of substrates.

The cross-dehydrogenative coupling of phenols and arenes 61

with a directing group containing a pyridine N-oxide moiety

occurs in the presence of Cu(OAc)2 under air atmosphere to


98

Scheme 7: ortho-Acetoxylation and methoxylation of O-methyl aryl oximes, N-phenylpyrrolidin-2-one, and (3-benzyl-4,5-dihydroisoxazol-5-yl)methylacetate.

Scheme 8: Ruthenium-catalyzed ortho-acyloxylation of acetanilides.


99

Scheme 9: Acetoxylation and alkoxylation of arenes with amide directing group using Pd(OAc)2/PhI(OAc)2 oxidative system.

form coupling products with one (product 62) or two (product

63) equivalents of phenol (Scheme 13) [67]. The same directing

group was used for aerobic ortho-alkoxylation of arenes 64 in

the presence of CuCl and K2CO3 to afford monoalkoxylated

products 65 [68].

The 2-aminopyridine-1-oxide directing group was used in a rare

example of a cobalt-catalyzed oxidative alkoxylation of arenes

66 and alkenes 67 to afford products 68 and 69 under mild

contitions [69] (Scheme 14). The directing group can be

removed to obtain the corresponding benzoic acid 71 from the

cross-dehydrogenative coupling product 70 [69].

The majority of above mentioned directing groups cannot be

easily removed or modified, thus limiting the scope of possible

target products. To overcome these limitations, 2-pyridyldiiso-

propylsilyl (PyDipSi) [70,71] and 2-pyrimidyldiisopropylsilyl

(PyrDipSi) [72,73] directing groups were proposed by the

research group of Prof. V. Gevorgyan. The PyDipSi group was

used for selective monoacetoxylation or pivaloyloxylation of

the arene ortho-position [70,71], whereas the PyDipSi group

allowed to achieve one-pot sequential acyloxylation of both

ortho-positions affording, in particular, orthogonally protected

resorcinol derivatives 73 in one step starting from PyrDipSi

arenes 72 [72] (Scheme 15).

Pd(OAc)2 was used as catalyst with addition of AgOAc or

LiOAc; PhI(OAc)2 or PhI(OPiv)2 were used as oxidants and

sources of OAc and OPiv fragments, respectively, the syntheses

were performed in dichloroethane at 80 °C for 1–168 h. After

ortho-acyloxylation step silyl directing group can be removed

or substituted by a desirable moiety (-Aryl, -OH, -B(pin), -I,

etc.).

Another example of an easily modifiable directing group is an

N-nitroso moiety that can be easily reduced to amine employing

Fe/NH4Cl [74]. N-Nitroso directed C–H alkoxylation of arenes

74 was realized using the Pd(MeCN)2Cl2/PhI(OAc)2 oxidative

system to obtain products 75 [74] (Scheme 16).

1.2 Reactions involving C(sp3)-H bonds ofC-reagents with alkyl groupsIn some studies, directing groups were used to accomplish the

C–O coupling involving sp3-carbon atoms of C-reagents.

Scheme 17 presents the alkoxylation of CH-reagents 76, 77 and

the acetoxylation of CH-reagents 78. In these examples, the


100

Scheme 10: Alkoxylation of azoarenes, 2-aryloxypyridines, picolinamides, and N-(1-methyl-1-(pyridin-2-yl)ethyl)amides using the Pd(OAc)2/PhI(OAc)2 oxidative system.

Scheme 11: Acetoxylation of compounds containing picolinamide and quinoline-8-amine moieties using the Pd(OAc)2/PhI(OAc)2 system.


101

Scheme 12: (CuOH)2CO3 catalyzed oxidative ortho-etherification using air as oxidant.

Scheme 13: Copper-catalyzed aerobic alkoxylation and aryloxylation of arenes containing pyridine-N-oxide moiety.

Scheme 14: Cobalt-catalyzed aerobic alkoxylation of arenes and alkenes containing pyridine N-oxide moiety.


102

Scheme 17: Selective alkoxylation and acetoxylation of alkyl groups.

Scheme 18: Acetoxylation of 2-alkylpyridines and related compounds.

Scheme 15: Non-symmetric double-fold C–H ortho-acyloxylation.

Scheme 16: N-nitroso directed ortho-alkoxylation of arenes.

reaction occurs at the methylene group of the molecule (with

the sp3-carbon atom) rather than at the ortho-position of the

aromatic system, which is also adjacent to the directing group.

The Pd(OAc)2/PhI(OAc)2 oxidative system was employed in

the methoxylation of dimethylcarbamoyltetrahydrocarbazoles

76 [75] and the acetoxylation of compounds containing the

picolinamide directing group 78 [76,77]; the butoxylation of the

methylene group of 2-benzylpyridine 77 took place in the pres-

ence of the Cu(OAc)2/AgOTf/O2 system [47]. Cross-dehydro-

genative C–O coupling products 79–81 were obtained, despite

the potential possibility of the more profound oxidation of the

methylene group to the keto group.

The benzylic position of 2-alkylpyridines and related com-

pounds 82 was acetoxylated using the Pd(OAc)2/CuI catalytic

system in acetic acid under an oxygen pressure of 8 atm to

prepare products 83 (Scheme 18) [78].

The acyloxylation of methyl or methylene groups of

8-methylquinoline and its derivatives in the presence of the

Pd(OAc)2/PhI(OAc)2 [79] and Pd(OAc)2/ligand/O2 [80] oxida-

tive systems was also described.


103

Scheme 19: Acyloxylation and alkoxylation of alkyl fragments of substrates containing amide or sulfoximine directing groups.

In the above considered examples, the oxidative C–O coupling

occurs with the involvement of methyl or methylene groups

directly bonded to the aromatic ring. The C–O coupling reac-

tions involving unactivated alkyl groups of C-reagents are

considered below.

The amide moiety was most commonly employed as the

directing group. The alkoxylation of alkyl groups of

N-(quinolin-8-yl)amides 84 [81], picolinamides 85 [62], and

N-(2-pyridin-2-yl)propan-2-yl)amides 86 [63], the trifluoroacet-

oxylation of amides 87 [82], and the acyloxylation of com-

pounds containing the S-methyl-S-pyridylsulfoximine moiety

88 [83] were accomplished to prepare coupling products 90–95

(Scheme 19). In some cases, the latter reaction proceeds effi-

ciently even at room temperature. Iodine(III) compounds 89,

(diacetoxyiodo)benzene, or potassium persulfate served as

oxidants. The former three reactions are applicable to a broad

range of substrates and alcohols. The drawback is that a large


104

Scheme 20: Palladium-catalyzed double sp3 C–H alkoxylation of N-(quinolin-8-yl)amides for the synthesis of symmetric and unsymmetric acetals.

Scheme 21: Copper-catalyzed acyloxylation of methyl groups of N-(quinolin-8-yl)amides.

excess of alcohols is required. Unlike the alkoxylation of

84–86, the trifluoroacetoxylation of 87 was performed with

structurally simple amides. This method is applicable only to

α-disubstituted amides. The reactions with amides containing

hydrogen in the α-position give coupling products 93 in

substantially lower yields [82]. Similar limitations are encoun-

tered when performing the acyloxylation of S-methyl-S-pyridyl-

sulfoximines 88.

Using the N-(quinolin-8-yl)amide directing group Pd(OAc)2

catalyzed the double sp3 C–H alkoxylation of 96 for the syn-

thesis of symmetric acetals 97 and unsymmetric acetals [84]

(Scheme 20). The method demonstrates good functional group

tolerance and the applicability in the synthesis of products

containing α-hydrogen next to an amide moiety.

Copper-catalyzed acyloxylation of methyl groups of N-(quin-

olin-8-yl)amides 99 was achieved [85] (Scheme 21, product

100); the reaction requires higher temperatures (170 °C) then

Pd(OAc)2 catalyzed alkoxylation of analogous substrates in the

presence of iodine(III) oxidants (50–130 °C, Scheme 19 and

Scheme 20). Under similar conditions methyl groups of

N-(quinolin-8-yl)amides were acetoxylated using the Cu(OAc)2

catalyst (50 mol %) and AgOAc (3 equiv) as acetoxylating

agent.

The acetoxylation of alkyl groups of O-acetyl oximes 102

taking place in the presence of the Pd(OAc)2/PhI(OAc)2 system

in a AcOH/Ac2O mixture affords products 103 (Scheme 22)

[56]. The acylation of oxime 101 and the C–H acetoxylation of

102 are performed as a one-pot operation. In the acetoxylation

of alkyl groups, the methyl group is more reactive than the

methylene one.

The O-methyl oxime [54,86] or pyridine moieties [86] were

employed as directing groups in the acetoxylation of alkyl

groups using the Pd(OAc)2/PhI(OAc)2 system. The synthesis

was carried out at 80–100 °C during a period of time from

5 min to 12 h in acetic acid, a 1:1 AcOH/Ac2O mixture, or

dichloromethane. More recently, a Pd(OAc)2/NaNO3/O2

catalytic system was developed for acetoxylation of

unactivated sp3-C–H bonds using the same directing groups

[87]. Air or oxygen (1 atm) played a role of terminal oxidant

and sodium nitrate was a redox co-catalyst; the reactions were

performed in a AcOH/Ac2O solvent mixture at 100–110 °C for

18 h.

The oxazole moiety also acts as the directing group in the

acetoxylation of alkyl groups with Pd(OAc)2/AcOOt-Bu or

Pd(OAc)2/lauroyl peroxide oxidative systems; in these reac-

tions acetic anhydride served as the source of the acetoxy


105

Scheme 22: One-pot acylation and sp3 C–H acetoxylation of oximes.

Table 2: Transition metal salt-catalyzed oxidative coupling of primary alcohols or aldehydes with alcohols using compounds with C=C, C=O, andC–Hal bonds as oxidants.

CH-reagent R2 (amount ofR2OH)

Catalyst Oxidant Conditions; yields Ref.

R1CH=O,R1CH2OH

Me(excess)

RuH2(CO)(PPh3)3/4,5-bis(diphenyl-phosphino)-9,9-dimethylxanthene

MeOH/PhMe (1:1),4–48 h, 110 °C;74–95%

[90,91]

R1CH2OH Me(excess)

[CpIrCl2]2/2-methylaminoethanol/Cs2CO3

acetone, 24 h, roomtemperature;23–92%

[92]

R1CH=OMe, Et, iPr,s-Bu, CH2CF3(excess)

Pd(OAc)2/2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl/K2CO3

acetone/R2OH,2–24 h, roomtemperature–50 °C;7–99%

[93]

R1CH=O Et(excess)

Pd(PPh3)4/K2CO3 BnCl or BnBr EtOH, MW, 30 min,90 °C; 65–93%

[94]

R1CH=O n-alkyl, sec-alkyl,Bn, etc. (1 equiv)

PdCl2(PPh3)2/K2CO3 BnCl THF, 20 h, 50 °C;72–99%

[95]

groups [88]. Recently, Cu(OAc)2-mediated аcetoxylation of

unactivated methyl fragments using the N-(quinolin-8-yl)amide

directing group and AgOAc as oxidant was achieved without a

Pd catalyst [89].

2 Aldehydes and alcohols as C-reagents incross-dehydrogenative C–O couplingThere are numerous reactions, in which aldehydes are involved

in the oxidative C–O coupling with alcohols under the action of

oxidizing agents to form esters. In some cases, primary alco-

hols are used instead of aldehydes. It is commonly assumed

that, under these reaction conditions, primary alcohols are

oxidized to aldehydes followed by cross-dehydrogenative C–O

coupling. These processes with aldehydes or primary alcohols

as C-reagents giving esters are often referred to as oxidative

esterification.

2.1 Transition metal salt-catalyzed reactions usingcompounds with C=C, C=O, and C–Hal bonds asoxidantsOne of the types of the oxidative esterification is based on the

hydrogen transfer catalyzed by transition metal complexes. In

these reactions, compounds with a double bond or a C–Hal

bond act as oxidants (hydrogen acceptors). Examples of these

reactions with aldehydes or primary alcohols as C–H reagents

are given in Table 2.


106

Scheme 23: Possible mechanism of oxidative esterification catalyzed by N-heterocyclic nucleophilic carbene.

Crotononitrile, acetone, and benzyl halides were used as

oxidants. Ruthenium, iridium, and palladium complexes acted

as catalysts. In most cases, structurally simple alcohols, which

are taken in a large excess relative to the CH-reagent, served as

OH-reagents. The exception is a study [95], in which the

coupling was accomplished using an equivalent amount of

alcohol.

2.2 Oxidative systems based on noble metals andoxygenThe oxidative coupling of benzyl alcohols with aliphatic alco-

hols in the presence of Pd(II) salt/Ag(I) salt/base/oxygen system

was proposed [96,97]. In the study [97], phosphine ligands were

additionally employed. It is suggested that the coupling occurs

through oxidation of benzyl alcohol to benzaldehyde. Alcohols

(OH-reagents) are taken in a twofold molar excess [96] or are

used as solvents [96,97]; the reaction time is 20–40 h at

45–80 °C. The aerobic oxidative coupling of aldehydes or pri-

mary alcohols as CH-reagents with low-molecular-weight alco-

hols was performed in the presence of heterogeneous catalysts,

such as Au/TiO2 [98-100], Au/β-Ga2O3 [101], Au/polymer

[102], and AuNiOx/SiO2-Al2O3-MgO [103]. In all the above-

mentioned processes in the presence of heterogeneous catalysts,

low-molecular-weight alcohols are used as the solvents or are

taken in a large excess relative to the CH-reagent.

2.3 Reactions catalyzed by N-heterocyclic carbenesN-heterocyclic nucleophilic carbenes 105 have found use for

the oxidative esterification. Scheme 23 shows, in a simplified

way, the proposed mechanism of this type of cross-dehydro-

genative C–O coupling [104-113]. The aldehyde is subjected to

the attack of N-heterocyclic nucleophilic carbene 105, which

can be generated from the corresponding azolium salt 104. The

resulting intermediate 106 is oxidized to 107 followed by the

nucleophilic attack by the alcohol to form ester 108.

The mechanism of the aerobic oxidative coupling of benzalde-

hyde with methanol in the presence of the 4-ethyl-1-methyl-1H-

1,2,4-triazolium iodide/DBU system was studied in detail [114].

It was shown that the reaction proceeds via another mechanism,

involving the formation of 2-hydroxy-1,2-diphenylethanone

from benzaldehyde followed by the oxidation of this intermedi-

ate to 1,2-diphenylethanedione.

Scheme 24 shows an example of the oxidative coupling of alde-

hydes and alcohols, in which CH- and OH-reagents are taken in

equivalent amounts [104]. Thiazolium salt 109 combined with

triethylamine acted as the catalyst, and azobenzene 110 served

as the oxidizing agent. Esters 111 were prepared in 16–97%

yield.

The selective oxidative coupling of aldehydes 112 and alcohols

113 in the presence of amines 114 using 1,4-dimethyl-

triazolium iodide (115), DBU and quinone 116 afforded esters

117. This method was also applied to the synthesis of esters

117a–c from amino alcohols (Scheme 25) [115,116].

Different conditions were proposed for the cross-dehydrogena-

tive C–O coupling of aldehydes with alcohols and phenols

catalyzed by N-heterocyclic carbenes (118, 129) or their precur-

sors, azolium salts (119–128), combined with bases (Table 3;

the publications are summarized in chronological order). In

most cases, alcohols were taken in an excess.

The oxidative esterification in the presence of azolium salts

119, 121, and 125 was performed with primary alcohols instead


107

Scheme 24: Oxidative esterification employing stoichiometric amounts of aldehydes and alcohols.

Scheme 25: Selective oxidative coupling of aldehydes with alcohols in the presence of amines.

of aldehydes (Table 4). It is supposed that under the reaction

conditions primary alcohol is initially oxidized to aldehyde

without the participation of N-heterocyclic carbene

[106,113,123,124].

The oxidative esterification of aldehydes with racemic mixtures

of alcohols catalyzed by chiral N-heterocyclic carbenes was

used for the kinetic separation of enantiomers of alcohols

[125,126].

2.4 Reactions using halogen-containing oxidativesystemsThis section considers cross-dehydrogenative C–O coupling

reactions with aldehydes and primary alcohols as C-reagents in


108

Table 3: Oxidative esterification of aldehydes with alcohols.

R1 R2 Oxidative system Conditions; yield Ref.

Ar n-alkyl, iPr

118 (1 mol per mol of aldehyde), generatedin situ

R2OH, bp; 17–75% [117]

n-alkyl, sec-alkyl, t-Bu,etc.

Me, n-Pr, cyclohexyl,CH2CH2SiMe3,CH2CCl3,CH(Me)COOMe 119 (10 mol %)

DBU (1.1 mol per mol of aldehyde)MnO2 (5 mol per mol of aldehyde)

R2OH (5 mol permol of aldehyde)CH2Cl2, roomtemperature;56–99%

[105,106]

Ar Me, iPr, Ph, etc.

120Cs2CO3 (1.5 equiv), air

R2OH (3 equiv)cyclohexane, 25 °C,10 h; 34–80%

[118]

Ar, PhCH=CH,cyclohexyl

Ar’

121 (5 mol %)Pd(OAc)2 (5 mol %)Na2CO3 (4 equiv)air

aldehyde/phenol(3:2)xylene, 100 °C,24–48 h; 25–99%

[119]


Ar’ 121 (20 mol %)Fe(OTf)2(20 mol %)t-BuOK (1 equiv)air

aldehyde/phenol(1:1)dioxane, 90 °C, 24h;15–89%

[107]


Me, Et, Bn, 2-PhEt,s-Bu, allyl,propargyl,4-pentinyl, etc.

122 (10 mol %)DBU (1 equiv)electrochemical oxidation, Bu4NBr(30 mol %)

aldehyde/alcohol(1:1.1)MeCN, roomtemperature;60–97%

[108]

Ar Me, Et, Pr, iPr

123 (3 mol per mol of aldehyde)

aldehyde/alcohol(1:3)50–60 °C;50–90%

[120]

PhCH2CH2 Bn, allyl

124 (5 mol %)NHEt2 (1.1 equiv)Et3N (1.2 equiv)N-chlorosuccinimide (1 equiv)

aldehyde/alcohol(1:2)CH2Cl2, roomtemperature;83–87%

[121]


109

Table 3: Oxidative esterification of aldehydes with alcohols. (continued)

Ar Me, Et, iPr, allyl,propargyl, Bn

125 (10 mol %)DBU (20 mol %)oxygen

aldehyde/alcohol(1:1.2)THF, 25 °C;63–82%

[109]


Me, Et, n-Pr, Bn,allyl, propargyl

126 (2.5 mol %)[Ru(2,2’-bipyrazine)3](PF6)2 (5 mol %), air

aldehyde/alcohol(1:10)MeCN, roomtemperature;15–81%

[110]

Ar, PhCH2CH2 Me, Bn, allyl,CH2CCl3

127 (15 mol %)DBU (110 mol %)air

THF/ROH (1:1) or3 equiv ROH in THF,room temperature;15–94%

[114,122]

Ar, n-alkyl n-alkyl, Bn,CH2CH2NEt2

128 (solvent)DBU (1 equiv), Cs2CO3 (3 equiv), MnO2(3 equiv)

aldehyde:alcohol(1:3)25 °C, 24 h;25–91%

[111]

Ar, CH=CHPh,CH=CH(2-C6H4OMe)

Bn, CH=CHPh, Et,Ph, (CH2)3Ph

129 (5 mol %)TEMPO (2 equiv)

aldehyde:alcohol(1:1.5)toluene, 100 °C,4–6 h60–86%

[112]

which halogens and their compounds, for example, molecular

iodine, the Bu4NI/t-BuOOH system, organic iodine(III or V)

compounds, bromides combined with oxidants, hypochlorite,

and so on, acted as oxidants.

The oxidative coupling of primary alcohols 130 with methanol

or trifluoroethanol and the oxidative coupling of aldehydes 132

with structurally diverse alcohols 133 were performed using

molecular iodine in the presence of potassium carbonate

(Scheme 26) [127]. In the former case, methanol or trifluoro-

ethanol served as the solvent. In the latter case, the reaction was

carried out in tert-butanol; the amount alcohol was nearly

equivalent relative to aldehyde. It is suggested that the reaction

proceeds through the formation of hemiacetal from alcohol and

aldehyde, which is oxidized by iodine to ester 131 or 134. In the

coupling of two alcohols, one of them is initially oxidized to

aldehyde by iodine.Scheme 26: Iodine mediated oxidative esterification.


110

Table 4: Oxidative esterification using primary alcohols as C-reagents.

R1 R2 Oxidative system Conditions; yield Ref.

substituted vinyl,alkynyl, Ar, Bn, etc.

Me, Bu, iPr,CH2CCl3,CH2CH2OMe,CH2CH2OTMS, etc.

125 (10–50 mol %), DBU (10–50 mol %),MnO2 (15 equiv)

R2OH or R2OH (5 equiv) intoluene, 23 °C; 65–95%

[113]

RCH=CH,PhCH=CMe,PhC≡C, Ar

Me, n-Bu, iPr

119 (10 mol %), DBU (10–110 mol %),MnO2 (15 mol per mol of R1OH)

R2OH or R2OH (3–5 equiv)in toluene, 23 °C; 73–95%

[106]

ArCH2, allyl Ar

121 (10 mol %), Pd(OAc)2 (5 mol %),Na2CO3 (0.5 mol per mol of phenol),oxygen

R1OH/R2OH (3:2)xylene, 130 °C, 36 h;35–95%

[123]

ArCH2, allyl, Bu Ar 121 (10 mol %), [RuCl2(p-cymene)]2(5 mol %), Cs2CO3 (10 mol %), oxygen

R1OH/R2OH (3:2)xylene, 130 °C, 24 h;50–95%

[124]

In the oxidative coupling of aldehydes or primary alcohols, the

second reagent (alcohol) usually served as the solvent. The oxi-

dation was performed with halogen-containing reactants:

I2/KOH [128], KIcat/t-BuOOH [129], I2cat/PhI(OAc)2 [130],

I2/NaNO2 [131], NaBr/PhI(OAc)2 [132], LiBr/NaIO4/H2SO4

[133], Bu4NBr/NaOCl [134], NaOCl/AcOH [135], Py·HBr3

[136], N-bromosuccinimide/pyridine [137], N-iodosuccinimide/

K2CO3 [138], and N,N'-diiodo-N,N'-1,2-ethanediylbis(p-

toluenesulfonamide) [139].

The oxidative C–O coupling of benzyl alcohols 135 with alkyl-

arenes 136 took place under the action of the Bu4NI/t-BuOOH

system in the presence of NaH2PO4 [140]. It was proposed that

under the reaction conditions benzyl alcohol 135 is oxidized to

carboxylic acid 138, while alkylarene gives iodide 139; the

nucleophilic substitution between the carboxylate anion and

benzyl iodide affords coupling product 137 [140] (Scheme 27).

This mechanism differs from that proposed in the study [141].

According to the latter mechanism, the benzylic carbocation

rather than benzyl iodide is generated and this carbocation

undergoes nucleophilic attack by carboxylic acid. This mecha-

nism is confirmed by the fact that iodide is inert under the reac-

tion conditions. The conditions of the oxidative coupling used

in the study [140] differ from the conditions described in the

work [141]. However, it should be noted that the formation of

benzyl iodide was not experimentally confirmed in the study

[140], and the presence of this species was proposed based on

the literature data.

The coupling of methyl- and ethylarenes 141 with aromatic

aldehydes 140 was performed with the Bu4NI/t-BuOOH system

in the presence of an excess of either alkylarene 141 or alde-

hyde 140 [142]. It was supposed that the coupling occurs

through the generation of tert-butoxyl radicals, which abstract a

hydrogen atom from the benzylic position of the C-reagent to

form the C-radical, which is oxidized to the carbocation; in turn,

the aldehyde is oxidized to acid, which reacts with the carbo-

cation to give the target coupling product 142 (Scheme 28).

It was shown that under the reaction conditions, tert-butyl

perester is generated as an important intermediate from alde-

hyde and t-BuOOH, and this intermediate can serve as the

source of acid and tert-butoxyl radicals [142].


111

Scheme 27: Oxidative C–O coupling of benzyl alcohols with methylarenes under the action of Bu4NI/t-BuOOH system in the presence of NaH2PO4.

Scheme 28: Oxidative coupling of methyl- and ethylarenes with aromatic aldehydes under the action of Bu4NI/t-BuOOH system.

tert-Butyl peresters 144 were synthesized on a preparative scale

by the cross-dehydrogenative C–O coupling of aldehydes 143

with t-BuOOH in the presence of Bu4NI [143]. It is proposed

that tert-butyl peresters 144 are produced as a result of the

recombination of acyl radicals 145 and tert-butyl peroxide radi-

cals. The radical reaction mechanism was confirmed by the

experiment, in which acyl radicals generated from aldehyde 146

were trapped by the stable radical TEMPO, and trapping

product 147 was obtained in an almost quantitative yield

(Scheme 29).

The reaction of aldehydes 148a with ethers 149 in the presence

of Bu4NI and t-BuOOH generated corresponding α-acyloxy

ethers 150. Reactions between (hetero)aromatic aldehydes or

cyclohexanecarbaldehyde 148b with arylalkyl ketones 151

under similar conditions resulted in α-acyloxy ketones 152

[144] (Scheme 30). The plausible mechanism includes the for-

mation of tert-butyl peresters from the aldehydes.

N-Hydroxyimides 154 are efficient OH-reagents in the coupling

with aldehydes and primary alcohols. Their oxidative coupling

products, so-called activated esters 155, readily react with

nucleophiles, alcohols or amines, due to which they are

employed for the preparation of esters and amides. This type of

coupling was accomplished using iodine-containing oxidants.

The proposed reaction mechanism involves the nucleophilic

addition of N-hydroxyimides to aldehydes followed by the oxi-

dation of the resulting adduct to form the activated ester. For

example, the C–O coupling of aldehydes 153 with N-hydroxy-

imides 154 was carried out in the presence of the Bu4NHal/t-

BuOOH system (Hal = I or Br) [145]. This method is applic-

able to the oxidative coupling of aldehydes 156 with hexa-

fluoroisopropanol giving esters 157. One of the coupling

components is added in a twofold excess (Scheme 31).

A similar oxidative coupling reaction was performed with pri-

mary alcohols 158 as CH-reagents and N-hydroxyimides 154a,b

[146]. The coupling reaction with N-hydroxyphthalimide

(154b) was carried out using the NaI/aqueous t-BuOOH/KOH

system instead of Bu4NI/t-BuOOH in decane [146]. The

resulting activated esters 159 and 160 were isolated or used in

the one-pot reaction with amines to prepare amides

(Scheme 32).

The oxidative C–O coupling of alcohols and aldehydes 161–164

with N-hydroxysuccinimide (154a) was performed in the pres-


112

Scheme 29: Cross-dehydrogenative C–O coupling of aldehydes with t-BuOOH in the presence of Bu4NI.

Scheme 30: Bu4NI-catalyzed α-acyloxylation reaction of ethers and ketones with aldehydes and t-BuOOH.

Scheme 31: Oxidative coupling of aldehydes with N-hydroxyimides and hexafluoroisopropanol.


113

Scheme 32: Oxidative coupling of alcohols with N-hydroxyimides.

ence of (diacetoxyiodo)benzene [147] or iodoxybenzoic acid

(IBX) [148] as the oxidant. The authors hypothesized that the

reaction proceeds via the nucleophilic addition of N-hydroxy-

succinimide (154a) to aldehyde followed by the oxidation of the

adduct that formed with iodine(III) [147] or iodine(V) [148]

compounds to form products 165, 166 (Scheme 33).

Scheme 33: Oxidative coupling of aldehydes and primary alcoholswith N-hydroxyimides using (diacetoxyiodo)benzene or iodoxybenzoicacid (IBX) as oxidants.

In the study [149], activated esters were synthesized by the

oxidative coupling of aldehydes and N-hydroxysuccinimide in

the presence of (diacetoxyiodo)benzene, and these compounds

were subjected, without isolation, in the reaction with amines to

prepare amides. The reaction was performed at room tempera-

ture with N-hydroxysuccinimide (154a) taken in an equivalent

amount or a small excess relative to aldehyde. Iodoxybenzoic

acid (IBX) or the Co(OAc)2·4H2O/O2 system proved to be less

efficient in the coupling reactions compared with (diacet-

oxyiodo)benzene. According to the proposed radical mecha-

nism, N-hydroxysuccinimide (154a) adds to aldehyde to form

intermediate 167 followed by the hydrogen atom abstraction

from 167 by succinimide-N-oxyl radical 168 to give radical 169

and the oxidation of the latter resulted in the formation of

coupling product 170 (Scheme 34). Radical intermediates 168

and 169 were detected by ESR spectroscopy [149].

The oxidative coupling of aldehydes 171 with pivalic acid (172)

was performed using the TEMPOcat/t-BuOCl system; the

coupling products, unsymmetrical anhydrides 173, were

employed in the synthesis of esters and amides 174

(Scheme 35) [150].

It is suggested that pivalic acid 172 adds to aldehyde 171 fol-

lowed by the oxidation of the intermediate that formed with the

oxoammonium salt, which is generated from TEMPO and

t-BuOCl, to yield anhydride 173.

2.5 Oxidative systems based on transition metalsalts and peroxidesEsters 177 were synthesized by the oxidative C–O coupling of

aldehydes 175 with alkylarenes 176 using the Cu(OAc)2/t-

BuOOH system (Scheme 36) [151]. The coupling was

performed with toluene, xylenes, 1,3,5-trimethylbenzene, 2,4-

dichlorotoluene, and ethylbenzene.

α-Acyloxy ethers 180 were synthesized by the oxidative

coupling of benzyl alcohols 178 with ethers 179 (dioxane, tetra-

hydropyran, tetrahydrofuran, 1,2-dimethoxyethane) using

Cu(OAc)2/t-BuOOH system [152] (Scheme 37).


114

Scheme 34: Proposed mechanism of the oxidative coupling of aldehydes and N-hydroxysuccinimide under action of (diacetoxyiodo)benzene.

Scheme 35: Oxidative coupling of aldehydes with pivalic acid (172).

Scheme 36: Oxidative C–O coupling of aldehydes with alkylarenes using the Cu(OAc)2/t-BuOOH system.

Scheme 37: Copper-catalyzed acyloxylation of C(sp3)-H bond adjacent to oxygen in ethers using benzyl alcohols.


115

Table 5: Oxidative coupling of alcohols, aldehydes, or formamides with 1,3-dicarbonyl compounds or phenols in the presence of tert-butyl hydroper-oxide and copper salts.

C-reagent O-reagent Molar ratio C-reagent/O-reagent;conditions; yields

Ref.

R1 = Ar, n-pentyl, cyclohexyl,diethylmethyl

R2 = Me, Et, CH2Cl, PhR3 = Me, Et, OMe, OEt

1:1.1;CuBr (2.5 mol %),t-BuOOH (5.5 M in decane, 1.5 equiv),80 °C, 16 h; 57–89%

[154]

R12 = di-Me, di-Et, di-iPr, -(CH2)5-

R2, R4 = alkyl, Ph, -(CH2)4-; R4

may be H;R3 = alkoxy, BnO

R5 = Me, OMe, Ph, NHPhR6 = H, OMe, Br

Formamide as the solvent;CuBr2 or Cu(OAc)2 (5 mol %),t-BuOOH (70% aq, 1.5 equiv),80 °C, 3 h;62–86%

[155]

R12 = di-Me, di-Et, di-iPr, -(CH2)5-

R2, R4 = alkyl, Ph, -(CH2)4-; R4

may be H;R3 = alkoxy, BnO

R5 = Me, OMe, Ph, NHPhR6 = H, OMe, Hal

Formamide as the solvent;CuCl (1 mol %),t-BuOOH (70% aq, 6 equiv),70 °C, 15–30 min;61–99%

[156]

R1 = Ar, n-alkyl, cyclohexyl, CH2CH2Ph,

R4 = Me, OMe, OEt, OBn, Ph

2:1;Cu(OAc)2 (5 mol %),t-BuOOH (70% aq, 4 equiv),DMSO, 80 °C, 20 h;35–88%

[153]

In a series of works, the oxidative coupling of alcohols, alde-

hydes, or formamides with 1,3-dicarbonyl compounds or

phenols was accomplished in the presence of tert-butyl

hydroperoxide and copper salts (Table 5). In most cases, the

range of phenols applicable to the coupling is limited to

2-acylphenols. However, 2-(benzothiazol-2-yl)phenol was used

along with 2-acylphenols as the OH-reagent in the study [153].

In the coupling with aldehydes, the components are taken in a

nearly stoichiometric molar ratio. In the coupling with


116

Table 5: Oxidative coupling of alcohols, aldehydes, or formamides with 1,3-dicarbonyl compounds or phenols in the presence of tert-butyl hydroper-oxide and copper salts. (continued)

R12 = di-Me, di-Et, di-iPr, -(CH2)5-,

-(CH2)2O(CH2)2-R5 = H, Me, t-Bu, OMe, NEt2,Cl, Br, etc.

Formamide as the solvent;CuCl (1–2 mol %),t-BuOOH (70% aq, 6 equiv),80 °C, 15–90 min;26–99%

[157]

R12 = di-Me, di-Et R2 = Me, Ph, n-Pr, etc.

R3 = Me, OMe, OEt, Bn, allyl,etc.

Formamide as the solvent;CuO/α-Fe2O3/carbon nanotubes,t-BuOOH (70% aq, 1.5 equiv),80 °C, 4 h; 40–80%

[158]

1:1;CuO/α-Fe2O3/carbon nanotubes,t-BuOOH (70% aq, 1.5 equiv),DMSO, 80 °C, 10 h; 20–82%

[158]

formamides, the latter served as the solvent. In the studies

[155,156], 1,3-cyclohexanedione was used as the OH-reagent

along with 1,3-ketoesters. However, the reaction with 1,3-

cyclohexanedione gave products in low yields (19–26%). The

coupling of a number of substituted salicylaldehydes with

formamides was performed; the aldehyde group, which is prone

to oxidation, remaining intact [157].

The coupling of aldehydes and formamides with 2-substituted

phenols was carried out in the presence of heterogeneous cata-

lysts, such as CuO on α-Fe2O3-modified carbon nanotubes (a

magnetically separable catalyst) [158] and the metal-organic

framework Cu2(4,4’-biphenyldicarboxylate)2(4,4’-bipyridine)

[159].

Aldehydes were oxidized to esters in alcohols with V2O5/

H2O2aq/HClO4 [160], V2O5/percarbonate, sodium perborate/

HClO4 [161], VO(acac)2/H2O2aq [162], Cu(ClO4)2/t-BuOOH in

decane/InBr3 [163], Fe(ClO4)3/H2O2aq [164], γ-Fe2O3-SiO2-

supported heteropoly acids combined with H2O2aq [165], silica

gel-immobilized manganese phthalocyanine/H2O2aq [166], a

Ni(II) complex/H2O2aq [167], and ZnBr2/H2O2 [168].

The oxidative C–O coupling of aromatic aldehydes 181 with

cycloalkanes 182 was accomplished in the presence of the

Cu(OAc)2/t-BuOOH system to prepare products 183. This reac-

tion is unusual in that it involves the cleavage of four C–H

bonds, including unactivated C–H bonds of cycloalkane, and

the formation of two C–O bonds and one C=C double bond

[169]. The yields of products 183 were not higher than 53%.

However, the transformation is a rare example of the oxidative

coupling with the participation of inert CH-reagents,

cycloalkanes, to form a product, which can be subjected to

higher oxidation under the conditions of oxidative coupling

(Scheme 38).

Scheme 38: Oxidative C–O coupling of aromatic aldehydes withcycloalkanes.

Related oxidative C–O coupling reactions of alkanes with

methylarenes and carboxylic acids are discussed in sections 4.1

and 6, respectively. Different radical mechanisms were

proposed, the common feature is the generation of alkene via

the radical dehydrogenation of alkane.

2.6 Other oxidative systemsEsters were synthesized from aldehydes and methanol (6 equiv

excess relative to aldehyde) using pyridinium dichromate in

DMF [170]. Methyl esters were synthesized also from aromatic

aldehydes, α,β-unsaturated aldehydes, or allylic alcohols in the

presence of the DDQ (2,3-dichloro-5,6-dicyanobenzoquinone)/

amberlyst-15 system in a methanol/toluene mixture under

microwave irradiation [171]. Methyl, ethyl, and isopropyl

benzoates were prepared from benzaldehyde under irradiation

of its alcoholic solutions with a mercury lamp in an oxygen


117

Scheme 39: Ruthenium catalyzed cross-dehydrogenative coupling of primary and secondary alcohols.

atmosphere; esters were obtained in higher yields in the pres-

ence of catalytic amounts of HCl [172]. The formation of pri-

mary and secondary alcohol esters was observed after the

ozonolysis of a mixture of aldehyde and alcohol in a basic

medium [173]. The oxidative esterification of aldehydes was

performed using peroxides in the presence of Lewis acids or in

the absence of the latter; for example, using oxone in the pres-

ence of In(OTf)3 [174,175], oxone [174,176], Caro’s acid [177],

H2O2aq (30%) in the presence of CaCl2 or MgCl2 [178], H2O2aq

(50%) [179].

The oxidative coupling of structurally diverse aldehydes

(aromatic, α,β-unsaturated, etc.) with hexafluoroisopropanol

was carried out in the presence of the oxoammonium salt

(4-acetylamino-2,2,6,6-tetramethylpiperidine-1-oxoammonium

tetrafluoroborate) and pyridine [180]. The drawback of this

method is that it requires a rather complicated and expensive

oxidant.

An efficient aerobic cross-dehydrogenative coupling of alco-

hols and α-carbonyl aldehydes was achieved employing a

CuBr/pyridine catalytic system in toluene, only 1.5-fold

excess of alcohols over aldehydes was used [181]. The reaction

time is 18 h at 90 °C, the yields of α-ketoesters vary from 42 to

88%.

The unusual C–O cross-coupling of primary alcohols 184 with

secondary alcohols 185 in the absence of oxidants was

performed in the presence of ruthenium complex 186 as the

catalyst; the reaction afforded molecular hydrogen and unsym-

metrical ester 187 (Scheme 39) [182].

Cross-coupling products 187 were obtained in high yields; the

expected homocoupling of primary alcohols giving symmet-

rical esters and the dehydrogenation of secondary alcohols

yielding ketones were avoided.

3 Ketones and 1,3-dicarbonyl compounds asC-reagents in cross-dehydrogenative C–OcouplingMost of cross-dehydrogenative C–O coupling reactions

involving the α-position of carbonyl compounds (acetoxylation,

alkoxylation, sulfonyloxylation) are based on the use of iodine-

containing oxidizing agents. Transition metal salts, such as

copper and manganese salts, were less often employed for this

purpose.

3.1 Oxidative systems based on iodine compoundsIodine(III) organic compounds, including those generated in

situ from aryl iodides and peracids (for example, m-chloroper-

benzoic acid (MCPBA) and peracetic acid) are most commonly

employed in the oxidative coupling of OH-reagents with car-

bonyl compounds. Methods were developed for the sulfonyl-

oxylation of ketones, in which iodoarene is generated in situ by

the iodination of arene with molecular iodine [183] or NH4I

[184] in the presence of m-chloroperbenzoic acid.

In the presence of p-(difluoroiodo)toluene (188), β-dicarbonyl

compounds 189 undergo oxidative coupling with various

OH-reagents (Scheme 40), such as sulfonic acids (coupling

products 190), acetic acid (products 191), diphenyl phosphate

(products 192), and alcohols (products 193) [185]. The reac-

tions with alcohols are most slow. It is suggested that the oxida-

tive C–O coupling at the α-position of carbonyl (or β-dicar-

bonyl) compounds and various OH-reagents in the presence of

iodine(III) compounds proceeds through an ionic mechanism.

Thus, the electrophilic iodine atom attacks the enol of carbonyl

compound 194 followed by the replacement of the iodine-

containing moiety in intermediate 195 by the O-nucleophile to

form C–O coupling product 196 (Scheme 40).

Table 6 presents other examples of C–O coupling reactions at

the α-position of carbonyl compounds based on the oxidation


118

Scheme 40: Cross-dehydrogenative C–O coupling reactions of β-dicarbonyl compounds with sulfonic acids, acetic acid, diphenyl phosphate, andalcohols using p-(difluoroiodo)toluene.

Table 6: C–O coupling reactions at the α-position of carbonyl compounds mediated by iodine(III) compounds or iodoarenes in the presence of perox-ides.

C-reagent O-reagent Oxidative system Conditions; yields Ref.

β-diketones, β-keto esters MeSO3H (1 equiv) PhIO CHCl3, reflux, 2 h;37–83%

[187]

β-diketones, β-keto esters MeOH or EtOH (as thesolvents)

PhIO, BF3·Et2O room temperature, 5 h;59–67%

[187]

ROH, R = iBu, CMe2Et,(CH2)2CF3,(CH2)3OBn, etc.

PhIO, BF3·Et2O CHCl3,room temperature, 5 h;yields were notreported

[188]

acetoxylation withPhI(OAc)2 (1.2 equiv)

PhI(OAc)2, Bu4NBr, KOH dioxane, roomtemperature, 1 h;74–87%

[189]


119

Table 6: C–O coupling reactions at the α-position of carbonyl compounds mediated by iodine(III) compounds or iodoarenes in the presence of perox-ides. (continued)

R1 = Ar, alkylR2 = H, alkyl, COOMe

RSO3H (1.1–5 equiv) PhI or poly(4-iodostyrene) (cat),MCPBA; KBrcat or TEMPOcat wasadded to oxidize alcohols

MeCN or CHCl3, 50 °C,5 h; 25–81%

[190]

R1 = Ar, alkyl,R2 = H, alkyl, CO2Et

p-TsOH (1.1–5 equiv.) polymer-immobilized iodobenzene,MCPBA (1.1–2.5 equiv), p-TsOH

50 °C, 9–16 h; 51–88% [191]

R1 = Ar, Et, t-Bu, etc.R2 = H, alkyl, etc.

acetoxylationwith Ac2O

PhI, 30% aq H2O2, Ac2O, BF3·Et2O 30 °C, 7 h; 32–86% [192]

R1 = Ar, alkyl,R2 = H, alkyl

p-TsOH (3–5 equiv) p-MeC6H4I (1 equiv), oxone(1.5 equiv),

MeCN, 60 °C;32–100%

[193]

R1 = Ar, alkyl,R2 = H, alkyl

RSO3H (1.5 equiv), R =Ar, alkyl, etc.

197 (0.1 equiv)MCPBA (1.5 equiv)

EtOAc, roomtemperature;8–41%

[186]

Scheme 41: Acyloxylation of ketones, aldehydes and β-dicarbonyl compounds using carboxylic acids and Bu4NI/t-BuOOH system.

with iodine(III) compounds or iodoarenes in the presence of

peroxides. Chiral iodoarenes, such as 197, served as the cata-

lysts in the asymmetric C–O coupling of sulfonic acids with

ketones. The enantiomeric excess of the product was not higher

than 58% due, in particular, to instability of the configuration of

the products, α-sulfonyloxy ketones, under the reaction condi-

tions, resulting in the partial racemization [186].

Good results were achieved in the oxidative C–O coupling of

ketones, aldehydes and β-dicarbonyl compounds 198 with

carboxylic acids 199 in the presence of the Bu4NI/t-BuOOH

system [194] (Scheme 41). The coupling can be accomplished

with a wide range of substrates and gives products 200 in high

yields, the C-component and the O-component being taken in a

ratio of 1:1. The reactions of aldehydes are similar to the reac-

tions of ketones, the aldehyde moiety remaining unchanged.

The authors hypothesized that the reaction proceeds through a

radical mechanism [194].

The Bu4NI/t-BuOOH system was employed also in the oxida-

tive coupling of carboxylic acids with β-keto esters [195].

The cross-dehydrogenative C–O coupling of alcohols 201 and

ketones 202 in the presence of the Bu4NI/t-BuOOH system was


120

Scheme 42: Acyloxylation of ketones using Bu4NI/t-BuOOH system.

accomplished to prepare α-acyloxy ketones 203 [196]. Coupling

products 203 were synthesized mainly from benzyl alcohols and

propiophenone (Scheme 42).

The authors proposed two radical reaction pathways, including

the formation of C-radicals from ketones. One of these path-

ways involves the formation of tert-butyl peresters from alco-

hols. This pathway is confirmed by the fact that, in the pres-

ence of Bu4NI, tert-butyl per(1-naphthylate) gives products 203

in the reaction with propiophenone.

3.2 Oxidative systems based on transition metalcompoundsIn addition to iodine compounds, transition metal salts, such as

copper and manganese salts, were used for the oxidative func-

tionalization at the α-position of carbonyl compounds.

N-Hydroxyimides and N-hydroxyamides 204 are involved in

the oxidative C–O coupling with 1,3-dicarbonyl compounds and

their hetero analogues, such as 2-substituted malononitriles and

cyanoacetic esters, 205 in the presence of oxidants based on

manganese, cobalt, and cerium [197]. The best results were

achieved with the use of Mn(OAc)3 and the Co(OAc)2(cat)/

KMnO4 system (Scheme 43). The yields of products 206 were

as high as 94%. It is supposed that the oxidant serves two func-

tions: the generation of N-oxyl radicals 207 from N-hydroxy-

imides or N-hydroxyamides 204 and the one-electron oxidation

of 1,3-dicarbonyl compounds via the formation of complex 208.

Apparently, the oxidative coupling of 1,3-dicarbonyl com-

pounds 209 with oximes 210 occurs via a similar mechanism

[198]. The reaction takes place in the presence of various

oxidants, such as KMnO4, Mn(OAc)2/KMnO4, Mn(OAc)3,

MnO2, Mn(acac)3, Fe(ClO4)3, Cu(ClO4)2, Cu(NO3)2, and

(NH4)2Ce(NO3)6. Twenty coupling products 211 were prepared

in yields of 27–92% using KMnO4, Mn(OAc)3·2H2O, or the

Mn(OAc)2/KMnO4 system. The syntheses do not require excess

amounts of reagents. The formation of N-oxyl radicals 212 and

207 from oximes and N-hydroxyimides was confirmed by ESR

spectroscopy [197,198].

The oxidative coupling of 1,3-dicarbonyl compounds [199] and

their hetero analogues [200] 213 with tert-butyl hydroperoxides

catalyzed by transition metal salts (Cu, Fe, Co, Mn) was

achieved. tert-Butyl hydroperoxide acts both as the oxidizing

agent and the O-component in the coupling. The best results

were obtained in the reaction catalyzed by Cu(ClO4)2·6H2O. It

was hypothesized that copper serves for the formation of the

reactive complex with 1,3-dicarbonyl compounds or their hetero

analogues, as well as for the generation of tert-butyl peroxide

radicals, which react with this complex to give coupling prod-

ucts 214 (Scheme 44).

The related peroxidation reactions with hydroperoxides

(t-BuOOH, PhMe2COOH) in the presence of transition metal

salts (cobalt, manganese, or copper) were performed with cyclo-

hexanone, 2-methylcyclohexanone, cyclohexene, 1-octene,

cumene, xylene, dimethylaniline, and dioxane [201].

The enantioselective oxidative coupling of 2,6-dialkylphenyl-β-

keto esters and thioesters 215 with tert-butyl hydroxycarbamate


121

Scheme 43: Cross-dehydrogenative C–O coupling of β-dicarbonyl compounds and their heteroanalogues with N-hydroxyimides, N-hydroxyamides,and oximes.

Scheme 45: Oxidative C–O coupling of 2,6-dialkylphenyl-β-keto esters and thioesters with tert-butyl hydroxycarbamate.

Scheme 44: Cross-dehydrogenative C–O coupling of β-dicarbonylcompounds and their heteroanalogues with t-BuOOH.

216 was performed using the Cu(OTf)2/chiral ligand 217/MnO2

system (Scheme 45) [202]. Apparently, product 218 is gener-

ated via an ionic mechanism involving the generation of elec-

trophilic nitrosocarbonyl intermediate 219 [202].

The improved version of that oxidative coupling method is

based on CuCl/Cu(OAc)2/ligand/air catalytic system; different

β-keto esters and hydroxamic acid derivatives can be used

[203].

The acetoxylation at the α’-position of α,β-unsaturated ketones

with Mn(OAc)3 was studied in detail. It is suggested that

manganese(III) acetate causes the generation of C-radicals from

ketones followed by the acetoxylation of these radicals. In this

reaction, Mn(OAc)3 [204-210] or acetic acid [209,210], which

was used as the co-solvent, can serve as the source of the

acetoxy group. The synthesis is usually performed in benzene.

In some cases, the α’-phenylation-α’-acetoxylation was

observed due, apparently, to the addition of the C-radical gener-

ated from ketone to benzene [204]. It was shown [209] that the


122

Scheme 46: α’-Acyloxylation of α,β-unsaturated ketones using KMnO4.

Scheme 47: Possible mechanisms of the acetoxylation at the allylic position of alkenes by Pd(OAc)2.

α’-acetoxylation of enones occurs with good selectivity in other

solvents, such as cyclohexane and acetonitrile, as well.

The acyloxylation of enones and aryl ketones 220 with

carboxylic acids occurs in benzene in the presence of KMnO4 to

form coupling products 221 (Scheme 46) [211]. Acids were

present in a large excess relative to ketones.

The synthesis was successfully accomplished, in particular,

with readily oxidizable formic acid; the corresponding formates

(for example, 221b) were obtained in 61–85% yield [211]. This

method was modified by using not carboxylic acids but their

mixtures with the corresponding anhydrides and was applied to

the α’-acyloxylation of α,β-unsaturated ketones with complex

structures with the aim of preparing analogues of natural com-

pounds [212].

4 Compounds with an allyl, propargyl, orbenzyl group as C-reagents in cross-dehy-drogenative C–O coupling4.1 Palladium- and copper-based oxidative systemsStudies on the acyloxylation at the allylic position of alkenes

with palladium(II) complexes started in the 1960s [213]. This

type of reactions was considered in more detail in reviews on

the palladium complex-catalyzed functionalization of allyl-

containing compounds [20,21].

It is suggested that the reaction proceeds through the cleavage

of the allylic C–H bond in 222 to form π-allyl–palladium com-

plex 223 followed by the nucleophilic attack of acetate to give

C–O coupling product 224a (Scheme 47) [214-217]. The alter-

native mechanism involves the acetoxypalladation of the double

bond in 222 resulting in the formation of intermediate 225 fol-


123

Scheme 49: Acyloxylation of terminal alkenes with carboxylic acids.

lowed by the elimination of HPdOAc to give product 224b

[213]. The first mechanism of the acetoxylation was confirmed

by the data on the acetoxylation of 1,2-dideuteriocyclohexene

[215], as well as by the detection of the π-allyl–palladium inter-

mediate [217]. However, it cannot be ruled out that the acetoxy-

lation can occur via the second mechanism under certain condi-

tions (Scheme 47) [21,213].

The palladium-complex-catalyzed acetoxylation of terminal

alkenes 226 under non-optimized conditions can afford a large

number of products: vinyl acetate 227 and methyl ketone 228

(Wacker reaction), E and Z isomers of linear allyl ethers 229

and 230, and branched allyl ether 231 (Scheme 48) [218].

Scheme 48: Products of the oxidation of terminal alkenes by Pd(II)/AcOH/oxidant system.

It was found that the selectivity of the reaction can be controlled

by changing the polarity of the solvent. The reactions of

terminal alkens with the Pd(OAc)2/benzoquinone system in a

mixture of DMSO and AcOH as the solvent selectively

produced linear E-allyl acetates in 50–65% yield [218]. The

reaction in acetic acid affords methyl ketone and vinyl acetate;

branched allyl ether is formed as the major product in the

CH2Cl2/AcOH system using the 1,2-bis (benzylsulfinyl)ethane

ligand [218].

The acyloxylation of terminal alkenes 232 with carboxylic acids

233 in the presence of 1,4-benzoquinone (BQ) as the oxidant,

vinyl phenyl sulfoxide ligand 234, and Pd(OAc)2 resulted in the

selective formation of branched allyl esters 235 [217]. This

reaction afforded linear esters 236 as byproducts. Apparently,

ligand 234 serves for the formation of the π-allyl–palladium

intermediate, and benzoquinone mediates the subsequent reduc-

tive elimination to give product 235 (Scheme 49) [217].

The Pd(OAc)2·[1,2-bis(phenylsulfinyl)ethane]-catalyzed

enantioselective acetoxylation of terminal alkenes was accom-

plished in the presence of a chiral Lewis acid; ee = 45–63%; the

reaction was performed with a small excess of AcOH

(1.1 equiv) in ethyl acetate using benzoquinone as the oxidant

[219].

The Pd(OAc)2-catalyzed acetoxylation of terminal alkenes

employing 4,5-diazafluorenone as the ligand can be performed

in the presence of oxygen as the oxidizing agent (1 atm). The

reaction gave linear allyl esters. The authors reported that this

ligand facilitates the reductive elimination of the allyl ester from

the π-allyl–palladium intermediate [220]. Linear E-allyl acetates

were synthesized also by the acetoxylation of terminal alkenes

with the PdCl2/NaOAc/AcOH/O2 system (5 atm) in N,N-

dimethylacetamide [221]. The Wacker reaction giving methyl

ketones occurs when using water instead of sodium acetate and

acetic acid, all other conditions being the same [221].

The coupling of terminal alkenes 237 with carboxylic acids

having complex structures 238 resulted in the selective forma-

tion of linear E-allyl esters 239 (Scheme 50) [222]. These reac-

tions afford a Z isomer of linear ester and branched allyl ester as

byproducts.

Similar results were obtained in the coupling of terminal

alkenes in the presence of lithium hydroxide as the base in

carboxylic acid acting as the OH-reagent or in a carboxylic

acid/acetonitrile mixture; the coupling was performed with

acetic, propionic, isobutyric, and pivalic acids [223].


124

Scheme 50: Synthesis of linear E-allyl esters by cross-dehydrogenative coupling of terminal alkenes wih carboxylic acids.

Scheme 51: Pd(OAc)2-catalyzed acetoxylation of Z-vinyl(triethylsilanes).

Scheme 52: α’-Acetoxylation of α-acetoxyalkenes with copper(II) chloride in acetic acid.

It was shown that the stereoselectivity of the Pd(OAc)2-

catalyzed acetoxylation of Z-vinyl(triethylsilanes) 240 can be

controlled using benzoquinone or (diacetoxyiodo)benzene as

the oxidant [224]. The reaction with benzoquinone affords the E

isomer of C–O coupling product 241, whereas Z isomer 242 is

formed as the major product in the case of (diacetoxy-

iodo)benzene (Scheme 51).

The α’-acetoxylation of α-acetoxyalkenes 243 with copper(II)

chloride in acetic acid was reported [225] (Scheme 52). Alkenes

free of the α-acetoxy group undergo trans-chlorination of the

double bond in the presence of CuCl2. α,α’-Diacetoxyalkenes

244 were synthesized from alkenes with the additional use of

the PdCl2/NaOAc/DDQ or Pd(OAc)2/benzoquinone/MnO2

reagents.

The oxidative acyloxylation at the allylic position of alkenes

and at the benzylic position of some alkylarenes 245 with

carboxylic acids 246 was accomplished in the presence of tert-

butyl hydroperoxide and mixed copper aluminum oxide as the

heterogeneous catalyst (Scheme 53) [226]. The catalyst was

prepared from CuCl2 and AlCl3·6H2O by the co-precipitation

from an aqueous solution in the presence of NaOH and

Na2CO3. Allyl esters 247 with various structures were synthe-

sized. The reactions were carried out taking alkene and

carboxylic acid in a ratio of 1:1 or in the presence of an excess

of alkene.

The intermolecular alkoxylation of methylheterocyclic com-

pounds 248, 249 using the CuBr/5,6-dimethyl-1,10-phenanthro-

line (5,6-Me2phen)/(t-BuO)2 oxidative system was shown by a

few examples [227]. The yields of coupling products 250, 251

were not higher than 37% (Scheme 54).

Methylarenes 252 were introduced into the oxidative C–O

coupling with β-dicarbonyl compounds 253 and phenols 254

[228]. The coupling afforded structures 255 and 256. The

method is applicable to phenols containing an aldehyde group,

which was not oxidized in the course of the reaction

(Scheme 55).


125

Scheme 53: Oxidative acyloxylation at the allylic position of alkenes and at the benzylic position of alkylarenes with carboxylic acids.

Scheme 55: Oxidative C–O coupling of methylarenes with β-dicarbonyl compounds or phenols.

Scheme 54: Copper-catalyzed alkoxylation of methylheterocyclic com-pounds using di-tert-butylperoxide as oxidant.

Oxidative C–O coupling of methylarenes 257 with cyclic ethers

258 and cycloalkanes 259 afforded α-acyloxy ethers 260 and

allyl esters 261, respectively. The reactions were accomplished

using the Cu(OAc)2/t-BuOOH oxidative system at low conver-

sions of methylarenes in the corresponding ether or cycloalkane

as solvent [229] (Scheme 56).

The proposed mechanism includes the copper-mediated genera-

tion of tert-butyl peresters from t-BuOOH and methylarenes

and the formation of final products via intermolecular coupling

of O-centered and C-centered radicals. Related oxidative C–O

coupling reactions of alkanes with aromatic aldehydes [169]

and carboxylic acids are discussed in sections 2.5 and 6, res-

pectively.


126

Scheme 56: Copper-catalyzed esterification of methylbenzenes with cyclic ethers and cycloalkanes.

Scheme 57: Oxidative C–O coupling of carboxylic acids with toluene catalyzed by Pd(OAc)2.

The 4-methyl group of 2,4,6-trimethylphenol was selectively

methoxylated in methanol in the presence of a copper(II) com-

plex and hydrogen peroxide [230]; the similar alkoxylation

reaction was performed with a stoichiometric amount of

copper(II) chloride [231].

The benzylation of aryl-, alkyl-, and cycloalkylcarboxylic acids

262 in toluene was accomplished in the presence of the

Pd(OAc) 2 /CF 3 SO 3 H/dimethy lace tamide /O 2 sys tem

(Scheme 57) [232]. Apparently, the reaction proceeds through

an ionic mechanism involving the cleavage of the C–H bond of

toluene with Pd(II). The product is formed either via the nucleo-

philic attack of carboxylic acid on Pd(II) complex 264 or as a

result of the reductive elimination of product 263 from com-

plex 265. It is supposed that dimethylacetamide promotes the

reoxidation of Pd(0) to Pd(II) with oxygen and suppresses the

aggregation of Pd(0), while trifluoromethanesulfonic acid facili-

tates the cleavage of the C–H bond of toluene through the for-

mation of cationic Pd(II) compounds [232].

The gas-phase aerobic acetoxylation of toluene with acetic acid

catalyzed by TiO2-, γ-Al2O3-, SiO2-, and ZrO2-supported

Pd–Sb particles was also reported [233-236].

4.2 Reactions catalyzed by tetrabutylammoniumiodideThe oxidative acyloxylation at the allylic position of alkenes

266 with carboxylic acids 267 was performed using the Bu4NI/

t-BuOOH system to prepare esters 268 [237]. Apparently, the

reaction proceeds through a radical mechanism, involving the

hydrogen atom abstraction from the allylic position of alkene

266 with the tert-butyl peroxyl or tert-butoxyl radical; the

authors suggested that the new C–O bond is formed as a result

of the cross-recombination of the allyl and carboxyl radicals

(Scheme 58) [237].

The coupling of carboxylic acids 269 with alkylarenes 270

occurs analogously to the coupling with alkenes (Scheme 59)

[141,237]. It is supposed that [Bu4N]+IO− or [Bu4N]+[IO2]−


127

Scheme 58: Oxidative acyloxylation at the allylic position of alkenes with carboxylic acids using the Bu4NI/t-BuOOH system.

Scheme 59: Cross-dehydrogenative C–O coupling of carboxylic acids with alkylarenes using the Bu4NI/t-BuOOH system.

generated in the Bu4NI/t-BuOOH system act as active oxidizing

agents; the fact that the reaction proceeds via the hydrogen atom

abstraction from the benzylic position to form the benzyl radical

is confirmed by a number of experiments, including the

trapping of the benzyl radical by the TEMPO radical [141].

The authors hypothesized that the benzyl radical is oxidized to

the benzyl cation, which undergoes nucleophilic attack by

the carboxylate anion to give cross-dehydrogenative C–O

coupling product 271. It was shown that 1-iodo-1-phenylethane

does not react with carboxylic acid under these reaction condi-

tions [141].

The oxidative C–O coupling of alkylarenes with aromatic

benzyl alcohols [140] and aldehydes [142] in the presence of

the Bu4NI/t-BuOOH system is considered in section 2.4

(Scheme 27 and Scheme 28).

The synthesis of symmetrical esters from methylarenes and of

unsymmetrical esters 274 from methylarenes 272 and

ethylarenes 273 in the presence of the Bu4NI/t-BuOOH system

was documented (Scheme 60). The reaction was accomplished

at low conversions of alkylarenes [238]. It was suggested that

methylarene is oxidized to carboxylic acid and reacts with the

benzylic carbocation generated from the second methylarene

molecule or from ethylarene through the hydrogen atom

abstraction followed by the oxidation.

The achievement of the cited study is that the authors succeeded

in performing the selective oxidative C–O cross-coupling of

methylarenes 272 with ethylarenes 273 and excluded side

processes, in which symmetrical esters could be generated from

methylarenes 272 (the formation of only small amounts of

symmetrical esters was observed; 2–5% yield), as well as the

oxidation of ethylarene 273 to aryl methyl ketone.

Phosphorylation of benzyl C–H bonds of alkylarenes 275 using

the Bu4NI/t-BuOOH oxidative system was documented to

afford products 277 (Scheme 61) [239]. In this process two


128

Scheme 60: Oxidative C–O cross-coupling of methylarenes with ethyl or isopropylarenes.

Scheme 61: Phosphorylation of benzyl C–H bonds using the Bu4NI/t-BuOOH oxidative system.

bonds (C–H and P–H) are cleaved and two new bonds (C–O

and P–O) are formed. The proposed mechanism includes benzyl

radical formation, oxidation of the latter to the benzylic cation,

and nucleophilic attack of phosphate which is formed by the

oxidation of the P–H bond in 276.

2,3-Disubstituted indoles 278 are acetoxylated in the presence

of the Bu4NI/PhI(OAc)2 system in a CH2Cl2/AcOH solvent

mixture. In most cases, the reaction resulted in the regioselec-

tive introduction of the acetoxy group at one of the two possible

positions resulting in the formation of product 279 or 280

(Scheme 62) [240].

The authors supposed that indoles undergo iodination (as exem-

plified by indoles 278a,b) with AcOI, which is generated from

PhI(OAc)2 and the iodide anion, through the formation of inter-

mediates 281a,b to give one of the two allyl iodides (282a or

282b); the nucleophilic attack of acetic acid or the acetate anion

(SN’ nucleophilic substitution) affords products 280a or 279a.

4.3 Reactions using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)The common feature of the cross-dehydrogenative C–O

coupling reactions that take place in the presence of DDQ is

that C-reagents should contain conjugated systems without

strong electron-withdrawing groups.

For example, the acetoxylation of alkylarenes 283 with DDQ in

acetic acid under ultrasound irradiation produced acetates 284

(Scheme 63) [241].

The ultrasound irradiation results in a considerable increase in

the selectivity of the reaction. The proposed mechanism

involves the generation of acetoxyl radicals and DDQ-H radi-

cals in the reaction of DDQ with acetic acid, and the latter radi-

cals abstract a hydrogen atom from the benzylic position of

alkylarene. The target product is formed as a result of the

recombination of benzyl and acetoxyl radicals. The reactions

with methylarenes were not performed; alkylarenes containing

electron-withdrawing substituents (Br, NO2) were not involved

in the oxidative coupling.

The oxidative coupling of diarylmethanes 285 with carboxylic

acids 286 takes place in the presence of the DDQcat/MnO2

system (Scheme 64) [242]. It is suggested that DDQ oxidizes

diarylmethanes to diarylmethyl cations, which react with

carboxylic acids to form esters 287. Manganese dioxide serves

for the oxidation of the reduced form of the catalyst DDQH2 to

DDQ. The drawback of this method is that the reaction center of

the C-component should contain two aryl groups and that a

fourfold excess of carboxylic acids (O-components) is required.

In the cited study, it was also reported that the coupling of

carboxylic acids 289 with 3-phenyl-2-propen-1-yl acetate (288)


129

Scheme 62: Selective C–H acetoxylation of 2,3-disubstituted indoles.

Scheme 63: Acetoxylation of benzylic position of alkylarenes usingDDQ as oxidant.

in the presence of DDQ gives allyl esters 290. DDQ was used

also for the acyloxylation at the benzylic position of dimethoxy-

arene 291 with carboxylic acids 292 to prepare esters 293

(Scheme 64) [243].

The benzylic position of β-phenylethylbenzamides was acetoxy-

lated with DDQ in acetic acid at 80 °C; the reaction readily

proceeds if the phenyl ring contains activating methoxy groups;

in the case of unsubstituted β-phenylethylbenzamide, the

conversion was incomplete [244]. (p-Isopropoxyphenyl)acetic

acid esters were stereoselectively acetoxylated with DDQ

despite the presence of an electron-withdrawing ester group in

the vicinity of the reaction center [245].

The oxidative C–O coupling of 1,3-diarylpropylenes 294 with

alcohols 295 was accomplished in the presence of DDQ [246];

the reaction was completed at room temperature in a period of

time shorter than one hour; a mixture of two isomeric coupling

products 296 was obtained in the case of different substituents

R1 and R2. Under similar conditions, the oxidative coupling of

1,3-diarylpropynes 297 with alcohols, phenols, and carboxylic

acids 298 afforded coupling products 299 (Scheme 65) [247].


130

Scheme 64: C–H acyloxylation of diarylmethanes, 3-phenyl-2-propen-1-yl acetate and dimethoxyarene using DDQ.

Scheme 65: Cross-dehydrogenative C–O coupling of 1,3-diarylpropylenes and 1,3-diarylpropynes with alcohols.

A method was developed for the one-pot synthesis of

3-acyloxy-3-azido-1-aryl-1-propynes 302 from 3-chloro-1-aryl-

propynes 300 involving the acyloxylation of the methylene

group with carboxylic acids 301 under the action of the FeCl2/

DDQ system (Scheme 66) [248]. In the absence of iron salts,

the reaction is less efficient.

The cross-dehydrogenative C–O coupling reactions with the use

of oximes as O-reagents in the presence of DDQ were docu-

mented. 1,3-Diarylpropylenes 303, as well as (E)-1-phenyl-2-

isopropylethylene (306) are involved in the oxidative coupling

with oximes 304 and 307 in the presence of DDQ [249]. Under

similar conditions, oximes 310 are involved in the coupling

with isochromanes 309 [250] (Scheme 67).

It was suggested that carbocations are generated from

C-reagents under the action of DDQ, and these carbocations

undergo nucleophilic attack by oxime anions to form coupling


131

Scheme 66: One-pot azidation and C–H acyloxylation of 3-chloro-1-arylpropynes.

Scheme 67: Cross-dehydrogenative C–O coupling of 1,3-diarylpropylenes, (E)-1-phenyl-2-isopropylethylene and isochromanes with oximes.

products 305, 308, and 311 [249,250]. N-Hydroxyphthalimide

was also employed as the O-reagent, the yield of the coupling

product with isochromane was 62% [250]. DDQ was also used

in the acetoxylation at the benzylic position of p-alkoxyalkyl-

arenes and p-alkylphenols in acetic acid [251].

4.4 Reactions with N-hydroxyphthalimideThe oxidative C–O coupling of alkylarenes and related com-

pounds 312 with N-hydroxyphthalimide (154b) was performed

using CuCl/PhI(OAc)2 [252] or (NH4)2Ce(NO3)6 [253] as the

oxidizing agent (Scheme 68). The reaction in the presence of

the CuCl/PhI(OAc)2 system gives somewhat higher yields of

coupling products 313 compared with (NH4)2Ce(NO3)6, but it

requires higher temperature, a longer reaction time, and rela-

tively larger excesses of alkylarene. Apparently, the C–O bond

is formed as a result of the recombination of benzyl and phthal-

imide-N-oxyl radicals. Alkylarenes should be taken in an excess

amount to achieve high yields. The coupling of N-hydroxy-

phthalimide with ethers and alkenes proceeds in a similar

fashion [252].

It was reported that such oxidative coupling reactions with

NHPI and alkylarenes can proceed through the generation of

phthalimide-N-oxyl radicals from NHPI in the presence of

Pb(OAc)4; however, the yields of the products were not

published [254]. Later on, it was shown that cross-dehydrogena-


132

Scheme 68: Cross-dehydrogenative C–O coupling of alkylarenes and related compounds with N-hydroxyphthalimide.

Scheme 69: Acetoxylation at the benzylic position of alkylarenes mediated by N-hydroxyphthalimide.

tive coupling of NHPI with toluene proceeds in the presence of

a variety of oxidants, (NH4)2Ce(NO3)6, PhI(OAc)2, KMnO4,

Mn(OAc)3·2H2O, Pb(OAc)4, or Co(OAc)2/O2 to afford

N-benzyloxyphthalimide in yields of 11–75% [255]. The oxida-

tive coupling of NHPI with cyclohexene and cyclooctene using

NaIO4 in the CH2Cl2/H2O mixture in the presence of silica gel

was documented [256]; the coupling products were only

partially characterized (the NMR spectra of the crude com-

pounds were reported). In another study, the oxidative coupling

of NHPI with cyclic and acyclic alkenes was performed in the

presence of (NH4)2Ce(NO3)6, Pb(OAc)4, or anthraquinone; in

the case of metal-containing oxidizing agents, the radical reac-

tion mechanism is suggested along with the competitive ionic

mechanism; the reaction products and procedures for their syn-

thesis were not completely characterized [257].

The acetoxylation at the benzylic position of alkylarenes 314 in

acetic acid was performed using the N-hydroxyphthalimide/

iodine/nitric acid system; in some experiments, Co(OAc)2 was

employed as the cocatalyst (Scheme 69) [258]. Oxygen or nitric

acid acts as the oxidant.

In the presence of oxidizing agents, phthalimide-N-oxyl radi-

cals are generated from NHPI, and these radicals abstract a

hydrogen atom from the benzylic position of alkylarene 314;

the resulting benzyl radical is trapped by iodine to form iodide


133

Scheme 70: C–O coupling of methylarenes with aromatic carboxylic acids employing the NaBrO3/NaHSO3 system.

Scheme 71: tert-Butyl peroxidation of allyl, propargyl and benzyl ethers catalyzed by Fe(acac)3.

followed by the replacement of iodine by acetic acid to give

product 315. The proposed mechanism is confirmed by the fact

that the reactions of alkylarenes containing electron-with-

drawing substituents in the benzene ring afford benzyl iodides

rather than the corresponding acetates.

4.5 Other reactionsThe C–O coupling of methylarenes 316 with aromatic

carboxylic acids 317 in the presence of the NaBrO3/NaHSO3

system was described [259]. The synthesis was performed at

room temperature with aromatic carboxylic acids 317

containing both electron-donating and -withdrawing groups;

methylarene 316 (toluene or 3-ethoxytoluene), and carboxylic

acid 317 were taken in a ratio of 1:1. It was suggested that

benzyl bromides are formed in situ followed by the generation

of products 318 through the nucleophilic attack by carboxylic

acid (Scheme 70).

The tert-butyl peroxide group was introduced into allyl,

propargyl and benzyl ethers 319 in the presence of the

Fe(acac)3/t-BuOOH system [260]. It was suggested that a

hydrogen atom is abstracted from the CH-reagent by tert-

butylperoxyl and tert-butoxyl radicals generated from tert-butyl

hydroperoxide in the presence of Fe(acac)3; the resulting

C-radical is oxidized with Fe(III) to the carbocation, and the

latter is subjected to nucleophilic attack by tert-butyl hydroper-

oxide to give product 320 (Scheme 71).

The tert-butyl peroxidation at the allylic and benzylic positions

of cyclohexene and ethylbenzene with tert-butyl hydroperoxide

catalyzed by nanosized CeO2 proceeds with low yields [261].

The acetoxylation of alkylarenes in acetic acid was performed

using the CeO2/NaBrO3 [262], LiBr/NaIO4 [263], and

H5PV2Mo10O40/NaNO3 systems [264]. The methyl group of

1,2,3-trimethoxy-5-methylbenzene in acetic acid was acetoxy-

lated with 65% nitric acid [265]. N-Benzylphthalimides were

acetoxylated at the benzylic position with the N-bromosuccin-

imide/NaOAc/HOAc system by heating under reflux in

chlorobenzene for 12 h [266].

The alkoxylation [267,268] or acyloxylation [267,269] at the

allylic position of alkenes occurs in the presence of selenium-

based oxidizing agents, such as ArSeSeArcat./S2O82− [267,268]


134

Scheme 72: Cross-dehydrogenative C–O coupling of ethers with carboxylic acids mediated by Bu4NI/t-BuOOH system.

Scheme 73: Oxidative acyloxylation of dimethylamides and dioxane with 2-aryl-2-oxoacetic acids accompanied by the decarboxylation.

or SeO2 [269]. It is suggested that the reactions proceed through

electrophilic attack of selenium on the C=C double bond

[268,269].

5 Ethers, amines, and amides as C-reagentsin cross-dehydrogenative C–O coupling5.1 Reactions catalyzed by tetrabutylammoniumiodideThe Bu4NI/t-BuOOH system proved to be efficient in the

oxidative C–O coupling of ethers 322 with carboxylic acids 321

(Scheme 72) [270]. The coupling was performed with ethers

taken in a 20-fold excess relative to carboxylic acids. It is

supposed that the tert-butoxyl radical, which is generated in the

Bu4NI/t-BuOOH system, abstracts a hydrogen atom from the

α-position of the ether, and the resulting C-radical is oxidized to

the carbocation, which reacts with the carboxylate anion to form

coupling product 323.

The Bu4NI/t-BuOOH system was applied to perform the

oxidative acyloxylation of dimethylamides 325 and dioxane

with 2-aryl-2-oxoacetic acids 324 accompanied by the

decarboxylation [271]. It was shown that, in the presence

of the Bu4NI/t-BuOOH system, 2-oxo-2-phenylacetic acid

(324a) is transformed into benzoic acid (328) followed by

the coupling of the latter with dimethylformamide to give prod-

uct 326a. Hence, it is supposed that arylcarboxylic acids are

intermediates in the synthesis of compounds 326 and 327

(Scheme 73).

The peroxidation of N-benzylamides and N-allylbenzamide 329

with the Bu4NI/t-BuOOH system (Scheme 74) was docu-

mented [272]. The tert-butyl peroxide group in products 330

can be replaced with an aryl, benzyl, or alkyl group by the reac-

tion with the corresponding Grignard reagent to afford products

331.


135

Scheme 74: tert-Butyl peroxidation of N-benzylamides and N-allylbenzamide using the Bu4NI/t-BuOOH system.

Scheme 75: Cross-dehydrogenative C–O coupling of aromatic carboxylic acids with ethers using Fe(acac)3 as catalyst and di-tert-butyl peroxide asoxidant.

Scheme 76: Cross-dehydrogenative C–O coupling of cyclic ethers with 2-hydroxybenzaldehydes using iron carbonyl as catalyst and tert-butylhydroperoxide as oxidant.

5.2 Reactions catalyzed by transition metal saltsThe oxidative coupling of aromatic carboxylic acids 332 with

ethers 333, which served as the solvents, was accomplished

using the Fe(acac)3/di-tert-butyl peroxide system to prepare

α-acyloxy ethers 334 (Scheme 75) [273].

In addition to aromatic carboxylic acids 332, the coupling was

performed with phenylacetic acid. The reaction takes place both

with ethers and cyclohexene. In almost all syntheses, one of the

following two C-reagents was used: 1,4-dioxane or 1,3-dioxo-

lane. It is interesting that the oxidative coupling of carboxylic

acids with 1,3-dioxolane occurs at the 4-poisition of dioxolane

(with one adjacent oxygen atom) rather than at the 2-position

(with two adjacent oxygen atoms), which is usually considered

to be more activated for the hydrogen atom abstraction.

The oxidative coupling of 2-hydroxybenzaldehydes 335 with

cyclic ethers 336 (1,4-dioxane, tetrahydrofuran) was performed

in the presence of the iron carbonyl/t-BuOOH system; in these

reactions, the ethers served as the solvents (Scheme 76) [274].


136

Scheme 77: Cross-dehydrogenative C–O coupling of ethers with β-dicarbonyl compounds and phenols using copper catalysts and tert-butylhydroperoxide as oxidant.

The reactions afforded products 337, leaving the aldehyde

group unchanged.

Ethers 338 were subjected to oxidative coupling with 1,3-

dicarbonyl compounds 339 and o-acylphenols 340 using the

copper salt/t-BuOOH system to prepare esters 341 and 342

(Scheme 77) [275].

In the proposed mechanism (Scheme 77) the reactive benzoyl

radical 344 derived from ether 343 by TBHP and a catalytic

amount of copper catalyst reacts with complex 345 by single

electron transfer to produce Cu(III) complex 346. Finally,

reductive elimination of intermediate 346 affords the desired

product 347 and the Cu(I) complex, which can be reoxidized to

Cu(II) by TBHP.

Scheme 78: Cross-dehydrogenative C–O coupling of 2-hydroxybenz-aldehyde with dioxane catalyzed by Cu2(BPDC)2(BPY).

Dioxane undergoes C–O oxidative coupling with 2-hydroxy-

benzaldehyde 348 in the presence of tert-butyl hydroperoxide

and the metal-organic framework (MOF) Cu2(BPDC)2(BPY)

(BPY = 4,4’-bipyridine, BPDC = 4,4’-biphenyldicarboxylate)

as the heterogeneous catalyst [276] (Scheme 78). The yield of


137

Scheme 79: Ruthenium chloride-catalyzed acyloxylation of β-lactams.

product 349 and the reactivity of related CH- and OH-reagents

under these reaction conditions were not discussed.

The ruthenium chloride-catalyzed acyloxylation of β-lactams

350 with an excess of acids as O-reagents was accomplished

using oxygen in the presence of acetaldehyde (Scheme 79). It

was hypothesized that the reaction proceeds through the genera-

tion of the Ru(V) oxo intermediate, which oxidizes the β-lactam

to the carbocation, and the latter is subjected to nucleophilic

attack by carboxylic acid to form substituted β-lactam 351

[277].

The tert-butyl peroxidation (product 354) or acetoxylation

(product 355) of the methylene groups of amides 352 and 353

adjacent to the nitrogen atom was performed in the presence of

a ruthenium-based catalyst using tert-butyl hydroperoxide or

peracetic acid, respectively [278] (Scheme 80).

Scheme 80: Ruthenium-catalyzed tert-butyl peroxydation amides andacetoxylation of β-lactams.

The α,β-diacetoxylation of N-arylpiperidines 356 with

PhI(OAc)2 affords products 357 in 20–46% yield. Apparently,

the reaction proceeds through the oxidation of amine to

enamine [279] (Scheme 81).

Examples of the alkoxylation of tertiary amines 358 and 359 by

their electrooxidation in alcohols were reported in the literature,

examples are given in Scheme 82 [280] (the ratio of products

360–363 was determined by gas chromatography).

Scheme 81: PhI(OAc)2-mediated α,β-diacetoxylation of tertiaryamines.

Scheme 82: Electrochemical oxidative methoxylation of tertiaryamines.

The coupling of N-hydroxyphthalimide with tetrahydrofuran

(70% yield) and isochromane (71% yield) takes place in the

presence of the CuCl/PhI(OAc)2 system; CH-reagents are taken

in a 10-fold excess (see section 4.4, Scheme 68) [252].

6 Other cross-dehydrogenative C–O couplingreactionsKetene dithioacetals 364 were subjected to acyloxylation in the

presence of the Pd(OAc)2/PhI(OAc)2 system and an excess of

carboxylic acids that served as the co-solvents to prepare prod-

ucts 365 (Scheme 83) [281]. The range of carboxylic acids used

in this reaction is limited to lower members of the series.

The oxidative coupling of enamines containing an electron-

withdrawing substituent 366 with carboxylic acids 367 occurs

efficiently in the presence of iodosobenzene (Scheme 84);

oxidative coupling products 368 were employed in the syn-

thesis of oxazoles [282].


138

Scheme 86: Proposed mechanism of the oxidative C–O coupling of actetanilide with O-nucleophiles in the presence of(di(trifluoroacetoxy)iodo)benzene.

Scheme 83: Cross-dehydrogenative C–O coupling of ketene dithioac-etals with carboxylic acids in the presence of the Pd(OAc)2/PhI(OAc)2system.

Scheme 84: Cross-dehydrogenative C–O coupling of enamides withcarboxylic acids using iodosobenzene as oxidant.

The oxidative alkoxylation [283], acetoxylation [283], and

tosyloxylation [284] of anilides 369 at the para-position were

accomplished using PhI(OOCCF3)2 in the presence of boron

trifluoride to prepare products 370–372, respectively

(Scheme 85); OH-reagents were taken in an excess.

The following mechanism was proposed. Anilide 373 is

subjected to electrophilic attack by (di(trifluoroacet-

oxy)iodo)benzene to form intermediate 374 followed by the

elimination of phenyl iodide from 374 to give cation 375, which

is subjected to nucleophilic attack (ROH, AcOH, or TsOH)

affording coupling product 376 (Scheme 86).

In the presence of PhI(OAc)2, aldehydes 377, anilines 378 and

alcohols undergo three-component coupling to produce alkoxyl-

Scheme 85: Oxidative alkoxylation, acetoxylation, and tosyloxylationof acylanilides using PhI(O(O)CCF3)2 in the presence of boron trifluo-ride.

Scheme 87: Three-component coupling of aldehydes, anilines andalcohols involving oxidative intermolecular C–O bond formation.

substituted N-arylimines 379 (Scheme 87). The preliminary

mechanism investigations revealed that the transformation

proceeds via imines as intermediates. Reactions were performed

in the medium of alcohol (OH-reagent) at room temperature

[285].

Various oxidants, such as (diacetoxyiodo)benzene [286-288],

PbCl2/HClO4, NaIO3/HClO4, Br2/N-iodosuccinimide, or

I2/30% aq H2O2 [289], were employed in the oxidative

coupling of phenols 380 with alcohols serving as the solvents

(Scheme 88) to prepare non-aromatic products 381 or 382.


139

Scheme 89: 2-Acyloxylation of quinoline N-oxides with arylaldehydes in the presence of the CuOTf/t-BuOOH system.

Scheme 90: Cross-dehydrogenative C–O coupling of azoles with primary alcohols.

Scheme 91: Oxidation of dipyrroles to dipyrrins and subsequent oxidative alkoxylation in the presence of Na3Co(NO2)6.

Scheme 88: Oxidative coupling of phenols with alcohols.

The 2-acyloxylation of quinoline N-oxides 383 with arylalde-

hydes 384 in the presence of the CuOTf/t-BuOOH system

afforded products 385 (Scheme 89) [290]. Benzaldehyde and

aromatic aldehydes containing electron-donating groups were

employed in the coupling. In addition to aromatic aldehydes,

the coupling was performed with cyclohexanecarbaldehyde. It

is suggested that the reaction proceeds through a radical mecha-

nism.

The oxidative coupling of primary alcohols 387 with azoles 386

was performed in the presence of the CuCl/(t-BuO)2 system

(Scheme 90) to prepare products 388 in 16–57% yield [291]. It

was hypothesized that the C–O bond is formed as a result of the

reductive elimination of coupling product 388 from the

copper(III) complex.

Alkoxylated heterocycles 391 were obtained in an excess of

alcohol in the presence of Na3Co(NO2)6 from dipyrrins 390,

which were generated from dipyrroles 389 (Scheme 91) [292].


140

1,2- and 1,4-naphthoquinones undergo alkoxylation in an alco-

holic medium in the presence of transition metal salts [293],

I2/CeCl3 [294], and HgO [295].

Alkanes are rarely used in the cross-dehydrogenative C–O

coupling because of low reactivity of C–H bonds. An example

is the trifluoroacetoxylation of cyclohexane, cycloheptane,

cyclooctane, and adamantane in trifluoroacetic acid in the pres-

ence of peracetic acid [296] or hydrogen peroxide [297-299]

with the addition of transition metal salts (Rh, Ru, Pd, Pt, Fe)

[296,297] or in the absence of metal compounds [298,299]; the

reactions were usually accomplished at room temperature for a

few hours.

Copper-catalyzed oxidative dehydrogenative carboxylation of

unactivated alkanes 393 with various aromatic acids 392 to

produce the corresponding allylic esters 394 was reported

recently [300] (Scheme 92). Mechanistic studies allowed

proposing a mechanism involving the generation of an allyl

radical via the formation of alkene from the starting alkane.

Related oxidative C–O coupling of aromatic aldehydes [169]

and methylarenes [229] with cycloalkanes using a Cu(OAc)2/t-

BuOOH system were considered in sections 2.5 (Scheme 38)

and 4.1 (Scheme 56), respectively.

Scheme 92: Oxidative dehydrogenative carboxylation of alkanes andcycloalkanes to allylic esters.

Acetoxylation of benzene was achieved employing Pd(OAc)2/

K2S2O8 oxidative system in AcOH/Ac2O mixture in the pres-

ence of a pyridinium-substituted pyridine ligand [301]

(Scheme 93). It was proposed that a key role for this cationic

ligand is to serve as a phase transfer catalyst to bring poorly

soluble S2O82− into contact with the Pd catalyst.

The electrochemical cross-dehydrogenative C–O coupling,

in which the O-reagent serves as the solvent, was described

in the literature. For example, the methoxylation of

methylarenes, amides, phenols, and other compounds was

performed in methanol using bases immobilized on a solid

support [302].

Scheme 93: Pd-catalyzed acetoxylation of benzene.

ConclusionThe analysis of the published data shows that oxidative C–O

coupling reactions attract an increasing interest of organic

chemists in recent years. In the past two decades, a consider-

able body of experimental data has been accumulated.

Compared with other types of coupling, the oxidative C–O

coupling is less well-studied despite the fact that the C–O–R

group is rather commonly present in various classes of organic

compounds, and a large number of diverse O-reagents are avail-

able for the coupling.

The drawback of the majority of the available methods for the

cross-dehydrogenative C–O coupling, which limits their appli-

cation to the coupling of two expensive complex compounds, is

the use of an excess of one of the starting components, the C- or

O-reagent. Besides, the synthesis often requires high tempera-

ture and long time to be completed.

The main goals in the development of the cross-dehydrogena-

tive C–O coupling are as follows: (1) a search for new reac-

tions involving various oxidative systems, C- and O-reagents;

(2) the development of methods, which do not require an excess

of the C- or O-reagent; (3) the development of methods based

on available, convenient, and safe oxidants; (4) a decrease in the

reaction temperature and time; (5) investigations of the mecha-

nisms of the oxidative coupling with the aim of predicting

conditions necessary for the efficient synthesis.

AcknowledgementsThis work was supported by the Russian Science Foundation

(Grant 14-23-00150).

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